Inconsistency, unfortunately, has long been one of Amazon’s hallmarks.
As our friend @oneida_books (among others) has pointed out over the years, there are some instances where Amazon will accept Retail Receipts for Supply Chain Provenance documentation, perhaps most-recently in this 032225 SAS post (link) to your now-closed “Letter of Authorization” thread.
Over the years, mentions of the acceptability of receipts, in various of the published policies have come and gone and come again, often enough with little if any easily-discernible obvious motivation for such changes.
Manny made an interesting 050626 post which I believe demonstrates some evidence of such inconsistency (emphasis mine):
To my way of thinking, a good deal of the ‘art’ for Selling on Amazon rests upon an ability to navigate the pitfalls found in this or that inconsistency.