When the bot is mentioned in a reply to a note by a "zap requester" (i.e. an iPhone user that writes a reply "@ZapOnBehalf 21"), it creates a zap request for the required amount, and mentions the requester username (in the zap content). It then sends the invoice to the requester in a DM to complete the zapping. No middleman needed in the payment, it's the same invoice directly from the zap recipient.