The ID returned from the API is the developer product ID, the shopId is a shop id, very similar to universe IDs, but useless. Right now, there is currently no way to get a product ID from a developer product ID (unless you search all pages of https://api.roblox.com/developerproducts/list?placeId={placeId}&page={page} and match the developer product ID with the product ID).
The website shows the actual product ID of the developer product (see here)
For any onlookers, api.roblox.com has been deprecated, use https://apis.roblox.com/developer-products/v1/universes/UNIVERSE_ID/developerproducts?pageNumber=1&pageSize=50 for getting a list of universe’s developer products and https://apis.roblox.com/developer-products/v1/developer-products/DEVELOPER_PRODUCT_ID for getting developer product by developer product ID not product ID.