update price and inventory on amazon via sp-api listings feeds

Steps

1. Authenticate to SP-API using Login with Amazon (LWA) — exchange your client credentials and refresh token for an access token via the LWA token endpoint.
2. Create a feed document by calling the `createFeedDocument` endpoint with `contentType` set to the appropriate feed MIME type (e.g., `text/tab-separated-values;charset=UTF-8` for flat file feeds or `application/json` for JSON-based feeds).
3. Upload your feed content (e.g., a price-and-quantity flat file or JSON inventory feed) to the pre-signed S3 URL returned in the `createFeedDocument` response using a PUT request with the correct `Content-Type` header.
4. Submit the feed by calling `createFeed` with `feedType` (e.g., `POST_PRODUCT_PRICING_DATA` for prices or `POST_INVENTORY_AVAILABILITY_DATA` for inventory) and the `feedDocumentId` from step 2.
5. Poll `getFeed` with the returned `feedId` until `processingStatus` is `DONE`, then retrieve the result feed document using `getFeedDocument` and download it from the signed URL to check for row-level errors.
6. For lower-latency inventory updates on a single listing, consider the `Listings Items` API's `patchListingsItem` endpoint as an alternative to feeds for individual SKU changes.