Which UPI ID prints as the QR code on my BytePhase invoices?

The one marked as the default handle for that business. It prints as a QR code on invoices and receipts. The first UPI ID you add becomes the default automatically, and marking another one as default moves the QR over to it. If you delete the default, the oldest remaining UPI ID takes over, so documents are never left without one. Adding or removing UPI IDs needs the business settings permission.