In the real flow the customer arrives from Xero: the "How to pay" button on the online invoice, or the link in the invoice email. These links open the same page directly.
New booking, nothing paid yet. Shows the deposit QR.
Deposit received. Shows the balance QR.
Deposit paid, then an extra night added on a child invoice. One balance QR covers both.
Bhutan SDF exception: deposit is half the trip plus the full SDF.