Anyone know why this has happened?
We are writing because you may have one or more listings for United States Postal Service (USPS) Forever stamps currently listed on Amazon. Effective January 1, 2020, Amazon will no longer permit the sale of Forever stamps on Amazon.com. Please remove your listings for Forever stamps before January 1, 2020. If you have not removed your listings for Forever stamps by that time, Amazon will remove them. Amazon will not be approving any listings for Forever stamps; please do not send documentation or ask for approval to create listings for Forever stamps.
If you have Forever stamps in one of Amazon’s fulfillment centers, you can learn more about creating removal orders here: https://sellercentral.amazon.com/gp/help/G200280650