As of the 13th of December 2021, I have been getting issues where the supplement feed in Google Merchant Center shows an error where the ID value does not match. While in fact, it’s correct.
Let’s go and check ourselves if it is indeed correct or not.
You need to check first to ensure the ID value is identical and case-sensitive. For example, if your product ID is shopify_US_305890525214_7499617337374, make sure you full cap US and lower case the rest in your supplement feed.
If you have confirmed that your supplement feed ID is the same as the primary feed ID, you are good.
You can check if your supplement feed works by going to Products > All Products > click on a product > scroll to the bottom and view the final attributes. If you can see your data has been applied, everything is working.
Make sure to wait at least half an hour after you processed the supplement feed before checking your final attributes.