Those are definitely wallets not users. The receipt event is sent from the pubkey of the LN wallet. What I’m talking about are the tags in the receipt event. In the NIP Appendix E it has this bullet point:

> - tags MUST include the p tag AND optional e tag from the zap request

I think the tags contain the pubkey of the person who requested the invoice. The p tag in the event is the pubkey you’re looking for.