I was going to suggest the same thing, using the classic site on the device, by accessing through a browser, not the app. I had to do it one time when my internet was down at home, and needed to pay for a combined invoice. No way to do it through the app, has to be a browser.

OP, tell them to access eBay through their phone browser, which will open the mobile view. Scroll down to the bottom and select classic site. Then it will be exactly the same as what it is on the desktop. It can be fiddly because you have to keep enlarging everything to be able to see it on the phone, but it's the easiest way to do it.