Get order-pay from checkout endpoint

Hello.
I have built a payment gateway for Piraeusbank (Greece). The problem I face is that the bank allows only one referral page, before starting the transaction. When its the first try, referral page is /checkout/… if transaction fails, Woocommerce has the cancel_order_url() or the two buttons (pay now, my account) which lead to checkout/order-pay/…
Unfrotunatelly, bank cannot accept the “checkout/order-pay/” as referral, as we already defined the /checkout/.
Any ideas or workarounds? Can I somehow call order-pay as checkout query string, or anything else?