I am still developing a conspiracy theory to explain this, but the essence is that Amazon has an AI bot that detects too many small item Prime purchases, and then auto-punishes you with a plausibly-deniable mechanism like sending empty packages. It will have the effect of discouraging me from buying $7 doohickies as individual purchases, which for them is probably a significant loss. (They used to have a Prime add-on escape clause, which basically required you to buy the $7 item as part of a larger purchase, so they could ship them all together. My conspiracy theory is that they are replacing that with this.)