During your Binance Pay wallet setup, you will need to select the order of your preferred currencies that you wish to make payments with. You can also visit the Binance Pay wallet tab and simply drag the tokens in the list to rearrange them in your desired order.
Your payments will be made in the order you have selected in Binance Pay. If the balance of your first preferred crypto is insufficient in the Binance Pay Wallet, the system will automatically move on to utilize the next crypto on your list to fulfill the total payment amount.