Can a customer pay for a whole order from their wallet balance?

Yes. When the customer ticks Use my wallet balance at checkout, Wcart shows how much comes from the wallet:

• If the balance covers the total, they see "order fully covered" and the order is placed without going through a payment gateway.
• If it doesn't, the wallet pays part of it and they pay the rest with another method.

The option only appears for signed-in customers when Allow paying with wallet at checkout is ticked on the Wallet plugin's Settings tab. If the order is later cancelled or refunded in full, the wallet amount goes back to the customer automatically.