Problem with payment method

I’m facing some issues with this plugin…

Here’s an example workflow.

1) i buy the gift card and I set the order as completed.
2) I apply the coupon code to the cart (not at the checkout) and then I go to the checkout. The total is less than the full value of the gift card, so I’ve still credit to use. The order completes without errors.
3) I make another order. This time I apply the coupon to the checkout page. I get the error about the selection of the payment.
4) I remove the coupon from the cart and… the coupon seems disappear, I cannot use it anymore. I don’t complete the order.
5) I buy another gift card and I set the order as complete.
6) I notice in your plugin, in the gift cards list, that a part of the previous coupon was used for the latest order, but it’s not true (and it was not applied in the latest order…)

Something weird… it seems there are multiple bugs here…