Initial work, copied from the Shopify provider

framework/saleor/utils/mutations/checkout-line-item-add.ts

@@ -0,0 +1,17 @@
+import { checkoutDetailsFragment } from '../queries/get-checkout-query'
+
+const checkoutLineItemAddMutation = /* GraphQL */ `
+  mutation($checkoutId: ID!, $lineItems: [CheckoutLineItemInput!]!) {
+    checkoutLineItemsAdd(checkoutId: $checkoutId, lineItems: $lineItems) {
+      checkoutUserErrors {
+        code
+        field
+        message
+      }
+      checkout {
+        ${checkoutDetailsFragment}
+      }
+    }
+  }
+`
+export default checkoutLineItemAddMutation