Take the money before the groom

Open a booking that hasn't happened yet and tap Take payment. You can do this the moment the booking exists.

Paying does not finish the job. The booking stays on your day and you still Start it as normal. The only thing that changes is that the app stops asking you for money: its card on your schedule says Paid, and the button that said Checkout now says Mark done.

That's on purpose. Paid and done are two different facts, and plenty of clients pay for next fortnight's groom today.

You can still move a paid booking to another time or day. Its services are locked, though — add any extras when you tap Mark done.

If the price changes on the day

Tap Mark done and add the extras as usual. The app works out what's left and offers to collect only the difference — you won't charge the whole booking twice. If it comes to less than they already paid, it tells you how much credit they're owed so you can sort it out with them.