Revolut offers an option to receive payments by sending someone a payment link. You define the amount of money you want to receive and it generates a unique link which you email to the person who just made a purchase on you opencart shop.
When they click on it, they will visit a website where they will provide some information and their credit card number, and voila! The amount you requested is now in your Revolut card. No charges and no fees.
So I modified "cash on delivery" payment method, renamed it to "Credit Card" and it simply informs the customer that once the order is placed, they will receive a Credit Card payment link. Job done!
The only minor issue here is that you have to generate the link and send it to the customer, which is a manual task. No big deal really, but ideally an opencart extension that talks to Revolut via API, and automatically generates the payment link for my Revolut account/amount, would be ideal.
I'm no developer so I'm just throwing this out there, in case someone wants to investigate if this is possible.