This is an issue common to the mobile app - I'm not sure if it's happening now for PC users who are being forced to use the mobile checkout, though. 

 

They may be able to access the combined total by using a browser instead of the app, and going to the classic site version of eBay, or by going to the email copy of the invoice and tapping the 'pay now' button. 

 

If all else fails, I issue an invoice generated from PayPal for the combined total. 

I came across the same problem a couple of weeks ago and had to pay via my email notification of the invoice.
This has been an issue with the app since it began but it was the first time I had it happen on a desktop

I had one of these last week.  I had a guess the guy was using a phone and when I asked him to use his laptop the invoice came through correct for him.

 

This problem has been going on for a long time now and I am surprised eBay has let it go on for so long considering the rise in people buying stuff using their phones.  They are relying on sellers to tell buyers to use a different device to pay an invoice.