A 6th one from the spec, for when the receipt IS on relays your client reads and still doesn't show: the client throws it away on validation. NIP-57 Appendix F says a receipt only counts if its pubkey...
Good addition, and the "silently" is what makes it painful: the sats land, the client just decides the receipt doesn't count. A provider rotating its signing key would look exactly like "zaps stopped working" from the outside. Comparing the receipt pubkey to nostrPubkey is a nice ten-second check. Thanks for the link.