It's quite a pickle you're in RBS. It could just be the card info form is really picky on formatting, the browser isn't good, website is truly broken, or it could be they really just don't take debit cards. I might be suggesting things you've already tried but I'll list as much stuff I can that I've faced with declined charges.
I'm not sure how you have your name on your card but it could be down to how they want that submitted. They may want the middle initial/name in a certain box. I'll use JOHN B DOE as an example. It could be the JOHN B is in the first name box and DOE in the last name box, or JOHN in the first name box and B DOE in the last. They may want it in all caps too. The card number could be picky too. Some want the spaces others not.
I did notice the site seems kind of broken in places with both Firefox and Chrome so it's possible it's the site or browser. I did notice something about UnionPay in their FAQ/Help section but when I clicked on it did not load. And while it is odd to suggest, maybe try to clear browser cookies/cache.
Another issue is your bank could be blocking it. Maybe give them a call and see if that's it. I'd also send JPDeliver another email. If that fails try contacting them through social media if they have some. However the reviews on their Facebook don't leave a good impression but they might answer there:
www.facebook.co...
To add to the bank issue you may want to check it's not putting authenticating charges through and messing with the balance or setting of fraud alert.
Unfortunately, the last two I hate suggesting. One is to accept they're just not going to take a debit card and either get a credit card or let the order go.
They state the card info has to match you so asking a relative or somebody to borrow their card isn't an option.
I wish you all the luck in getting this sorted out RBS.