Create BitPay invoice with orderId (not CartId)

[pieterpoorthuis]

The difficulty is that the module needs to validate the order before the BitPay invoice is created. Only this way it becomes possible to create a BitPay invoice with Order ID (instead of Cart Id).

Taken from this forum thread: https://www.prestashop.com/forums/topic/870582-bitpay-pre-payment-order-validation/