This is not a per-title nor a per-seller limit, nor is it related to Amazon Prime or FREE Shipping or whatever. It's across all titles and sellers, in any combination (including 3rd party sellers), with any form of shipping, Prime or non-Prime.

Talking with customer support over the weekend, they claimed it was an error and would be fixed today, but as of this posting, the four-per-week purchase limit is still active.

The specific error message (during checkout) is:

There was a problem with some of the items in your order (see below for more information):

You have reached the purchase limit for this item. We have changed the quantity to the maximum allowable.

If the quantity has been set to 0, please delete the item (below the item details) to proceed.

More info on the problem here: https://forums.stevehoffman.tv/threads/anyone-having-troubles-ordering-from-amazon.1143712/

The problem first appeared on Friday, April 22, 2022. If anyone at Amazon can get this fixed, that would be great.