A massage booked for this afternoon and a tour booked for next Tuesday are not the same commercial commitment, and charging them the same way annoys somebody either way.
Paid-now posts the charge at booking, fully taxed, which suits a treatment taken today. Posted-at-delivery writes nothing until the guest actually attends, which suits a tour a guest may cancel inside the window without paying.
Delivery is also when consumables come off stock. The oils, linens or equipment attached to a service deduct against the attendee count when the service is marked attended — not when it is booked, so a cancellation never has to restore anything.
A walk-in opens its own party and settles at the counter. An in-house guest rides an outlet party whose balance is posted to the room. Either way the guest bill foots without a second billing path.