> maintaining swift response times on an issue that isn't even theirs?

But the problem is theirs. Their site says it all:

"No setup fees, no monthly fees, no card storage fees, no hidden costs: you only get charged when you earn money."

Therefore, if you're having an issue with your checkout form and their API, it very much is their problem. You're not able to process payments, so you're not able to get paid. Because you're not getting paid, they're not getting paid.