G2A Pay doesn't allow any fees or taxes [WILL PAY FOR HELP]

Hello,

I'm trying to use G2A pay as one of my checkout options on OC 2.2. It works fine except whenever I add taxes/low order fee/handling fee, clicking 'confirm order' at the end of checkout takes me to a 'page not found' error page.

This issue goes away as soon as I remove the tax/extra fee. The issue does not occur with PayPal nor with a Bitcoin extension I have. It seems that only G2A Pay cannot handle any sort of additional fee in the checkout.

I'm willing to PayPal some money to anyone who provides a solution that fixes this. Perhaps there's some simple setting I've been overlooking.

Please note that I can't provide FTP access as there's too much sensitive data on the server.

Post or PM, thanks!