diff --git a/framework/bigcommerce/api/cart/add-item.ts b/framework/bigcommerce/api/cart/add-item.ts
new file mode 100644
index 000000000..8c83c4f7b
--- /dev/null
+++ b/framework/bigcommerce/api/cart/add-item.ts
@@ -0,0 +1,46 @@
+import { parseCartItem } from '../utils/parse-item'
+import getCartCookie from '../utils/get-cart-cookie'
+import type { CartHandlers } from '.'
+import { normalizeCart } from '@framework/lib/normalize'
+
+const addItem: CartHandlers['addItem'] = async ({
+  res,
+  body: { cartId, item },
+  config,
+}) => {
+  if (!item) {
+    return res.status(400).json({
+      data: null,
+      errors: [{ message: 'Missing item' }],
+    })
+  }
+  if (!item.quantity) item.quantity = 1
+
+  const options = {
+    method: 'POST',
+    body: JSON.stringify({
+      line_items: [parseCartItem(item)],
+      ...(!cartId && config.storeChannelId
+        ? { channel_id: config.storeChannelId }
+        : {}),
+    }),
+  }
+  const { data } = cartId
+    ? await config.storeApiFetch(
+        `/v3/carts/${cartId}/items?include=line_items.physical_items.options`,
+        options
+      )
+    : await config.storeApiFetch(
+        '/v3/carts?include=line_items.physical_items.options',
+        options
+      )
+
+  // Create or update the cart cookie
+  res.setHeader(
+    'Set-Cookie',
+    getCartCookie(config.cartCookie, data.id, config.cartCookieMaxAge)
+  )
+  res.status(200).json({ data: normalizeCart(data) })
+}
+
+export default addItem
diff --git a/framework/bigcommerce/api/cart/index.ts b/framework/bigcommerce/api/cart/index.ts
index 4ee668895..22adb57d4 100644
--- a/framework/bigcommerce/api/cart/index.ts
+++ b/framework/bigcommerce/api/cart/index.ts
@@ -1,7 +1,7 @@
+import { CartHandlers as CartHandlersCore } from '@commerce/api'
 import isAllowedMethod from '../utils/is-allowed-method'
 import createApiHandler, {
   BigcommerceApiHandler,
-  BigcommerceHandler,
 } from '../utils/create-api-handler'
 import { BigcommerceApiError } from '../utils/errors'
 import getCart from './handlers/get-cart'
@@ -14,14 +14,22 @@ import type {
   AddCartItemHandlerBody,
   UpdateCartItemHandlerBody,
   RemoveCartItemHandlerBody,
+  Cart,
 } from '../../types'
+import { APIHandler } from '@commerce/api/utils/types'
+import { CommerceAPI } from '..'
 
-export type CartHandlers = {
-  getCart: BigcommerceHandler<BigcommerceCart, GetCartHandlerBody>
-  addItem: BigcommerceHandler<BigcommerceCart, AddCartItemHandlerBody>
-  updateItem: BigcommerceHandler<BigcommerceCart, UpdateCartItemHandlerBody>
-  removeItem: BigcommerceHandler<BigcommerceCart, RemoveCartItemHandlerBody>
-}
+export type CartHandlers<
+  C extends CommerceAPI = CommerceAPI
+> = CartHandlersCore<
+  C,
+  {
+    getCart: APIHandler<C, CartHandlers<C>, Cart | null, GetCartHandlerBody>
+    addItem: APIHandler<C, CartHandlers<C>, Cart, AddCartItemHandlerBody>
+    updateItem: APIHandler<C, CartHandlers<C>, Cart, UpdateCartItemHandlerBody>
+    removeItem: APIHandler<C, CartHandlers<C>, Cart, RemoveCartItemHandlerBody>
+  }
+>
 
 const METHODS = ['GET', 'POST', 'PUT', 'DELETE']
 
diff --git a/framework/bigcommerce/api/index.ts b/framework/bigcommerce/api/index.ts
index 840645459..d11f168b8 100644
--- a/framework/bigcommerce/api/index.ts
+++ b/framework/bigcommerce/api/index.ts
@@ -104,7 +104,9 @@ export const provider = {
 
 export type Provider = typeof provider
 
-export class CommerceAPI<P extends Provider> extends CoreCommerceAPI<P> {
+export class CommerceAPI<
+  P extends Provider = Provider
+> extends CoreCommerceAPI<P> {
   constructor(readonly provider: P = provider) {
     super(provider)
   }
diff --git a/framework/commerce/api/endpoints/cart.ts b/framework/commerce/api/endpoints/cart.ts
index 32c73f264..7bcb2c46f 100644
--- a/framework/commerce/api/endpoints/cart.ts
+++ b/framework/commerce/api/endpoints/cart.ts
@@ -1,9 +1,9 @@
 import type { APIEndpoint } from '../utils/types'
 import { CommerceAPIError } from '../utils/errors'
 import isAllowedOperation from '../utils/is-allowed-operation'
-import type { APIProvider, CartHandlers } from '..'
+import type { CommerceAPI, CartHandlers } from '..'
 
-const cartApi: APIEndpoint<APIProvider, CartHandlers<any>> = async (ctx) => {
+const cartApi: APIEndpoint<CommerceAPI, CartHandlers<any>> = async (ctx) => {
   const { req, res, handlers, config } = ctx
 
   if (
diff --git a/framework/commerce/api/index.ts b/framework/commerce/api/index.ts
index 85079bb34..e2bc4f225 100644
--- a/framework/commerce/api/index.ts
+++ b/framework/commerce/api/index.ts
@@ -1,16 +1,22 @@
 import type { NextApiHandler } from 'next'
 import type { RequestInit, Response } from '@vercel/fetch'
 import type { APIEndpoint, APIHandler } from './utils/types'
+import type { Cart } from '../types'
 
-export type CartEndpoint = APIEndpoint<any, any, CartHandlers<any>, any>
+export type CartEndpoint = APIEndpoint<any, any, CartHandlers<CommerceAPI>, any>
 
-export type CartHandlers<Body extends { cartId: 'string' }> = {
-  getCart: APIHandler<any, CartHandlers<Body>, any, Body>
-  addItem: APIHandler<any, CartHandlers<Body>, any, Body>
-  updateItem: APIHandler<any, CartHandlers<Body>, any, Body>
-  removeItem: APIHandler<any, CartHandlers<Body>, any, Body>
+export type CartHandlersBase<C> = {
+  getCart: APIHandler<any, CartHandlersBase<C>, Cart | null, any>
+  addItem: APIHandler<any, CartHandlersBase<C>, Cart, any>
+  updateItem: APIHandler<any, CartHandlersBase<C>, Cart, any>
+  removeItem: APIHandler<any, CartHandlersBase<C>, Cart, any>
 }
 
+export type CartHandlers<
+  C extends CommerceAPI,
+  T extends CartHandlersBase<C> = CartHandlersBase<C>
+> = T
+
 export type Endpoints = CartEndpoint
 
 export type EndpointHandlers<E> = E extends APIEndpoint<any, any, infer T>
@@ -21,17 +27,12 @@ export type EndpointOptions<E> = E extends APIEndpoint<any, any, any, infer T>
   ? T
   : never
 
-export type CoreAPIProvider = {
+export type APIProvider = {
   config: CommerceAPIConfig
 }
 
-export type APIProvider<P extends CoreAPIProvider = CoreAPIProvider> = P & {
-  getConfig(userConfig?: Partial<P['config']>): P['config']
-  setConfig(newConfig: Partial<P['config']>): void
-}
-
 export class CommerceAPI<
-  P extends CoreAPIProvider = CoreAPIProvider,
+  P extends APIProvider = APIProvider,
   E extends Endpoints = Endpoints
 > {
   constructor(readonly provider: P) {
@@ -55,14 +56,14 @@ export class CommerceAPI<
     operations: EndpointHandlers<typeof context.handler>
     options?: EndpointOptions<typeof context.handler>
   }): NextApiHandler {
-    const provider = this
+    const commerce = this
     const cfg = this.getConfig(context.config)
 
     return function apiHandler(req, res) {
       return context.handler({
         req,
         res,
-        provider,
+        commerce,
         config: cfg,
         handlers: context.operations,
         options: context.options,
@@ -71,23 +72,6 @@ export class CommerceAPI<
   }
 }
 
-export function createAPIProvider<P extends CoreAPIProvider>(
-  provider: P
-): APIProvider<P> {
-  return {
-    ...provider,
-    getConfig(userConfig = {}) {
-      return Object.entries(userConfig).reduce(
-        (cfg, [key, value]) => Object.assign(cfg, { [key]: value }),
-        { ...this.config }
-      )
-    },
-    setConfig(newConfig) {
-      Object.assign(this.config, newConfig)
-    },
-  }
-}
-
 export interface CommerceAPIConfig {
   locale?: string
   commerceUrl: string
diff --git a/framework/commerce/api/utils/types.ts b/framework/commerce/api/utils/types.ts
index fcf2057dd..c971d64f8 100644
--- a/framework/commerce/api/utils/types.ts
+++ b/framework/commerce/api/utils/types.ts
@@ -1,21 +1,25 @@
-import type { NextApiHandler, NextApiRequest, NextApiResponse } from 'next'
-import type { APIProvider } from '..'
+import type { NextApiRequest, NextApiResponse } from 'next'
+import type { CommerceAPI } from '..'
 
-export type APIResponse<Data = any> = {
-  data: Data
-  errors?: { message: string; code?: string }[]
-}
+export type ErrorData = { message: string; code?: string }
+
+export type APIResponse<Data = any> =
+  | { data: Data; errors?: ErrorData[] }
+  // If `data` doesn't include `null`, then `null` is only allowed on errors
+  | (Data extends null
+      ? { data: null; errors?: ErrorData[] }
+      : { data: null; errors: ErrorData[] })
 
 export type APIHandlerContext<
-  P extends APIProvider,
-  H extends APIHandlers<P, Data> = {},
+  C extends CommerceAPI,
+  H extends APIHandlers<C> = {},
   Data = any,
   Options extends {} = {}
 > = {
   req: NextApiRequest
   res: NextApiResponse<APIResponse<Data>>
-  provider: P
-  config: P['config']
+  commerce: C
+  config: C['provider']['config']
   handlers: H
   /**
    * Custom configs that may be used by a particular handler
@@ -24,22 +28,22 @@ export type APIHandlerContext<
 }
 
 export type APIHandler<
-  P extends APIProvider,
-  H extends APIHandlers<P, Data> = {},
+  C extends CommerceAPI,
+  H extends APIHandlers<C> = {},
   Data = any,
   Body = any,
   Options extends {} = {}
 > = (
-  context: APIHandlerContext<P, H, Data, Options> & { body: Body }
+  context: APIHandlerContext<C, H, Data, Options> & { body: Body }
 ) => void | Promise<void>
 
-export type APIHandlers<P extends APIProvider, Data = any> = {
-  [k: string]: APIHandler<P, any, Data, any>
+export type APIHandlers<C extends CommerceAPI, Data = any> = {
+  [k: string]: APIHandler<C, any, Data, any, any>
 }
 
 export type APIEndpoint<
-  P extends APIProvider = APIProvider,
-  H extends APIHandlers<P, Data> = {},
+  C extends CommerceAPI = CommerceAPI,
+  H extends APIHandlers<C> = {},
   Data = any,
   Options extends {} = {}
-> = (context: APIHandlerContext<P, H, Data, Options>) => void | Promise<void>
+> = (context: APIHandlerContext<C, H, Data, Options>) => void | Promise<void>