I recently did a test on this myself, I went in and purchased a few item via the mobile phone eBay app. boy was  it a pain.....when you purchase the 1st item., a red warning thing comes up on the screen

**You have not paid for this item**

This warning comes up on every item you buy, it also brings up the screen with payment options on it...it is a challenge to get out of that screen to get back to the sellers store to purchase more.

Then when the seller sends a combined invoice and you go to pay this message comes up on the screen

You can't pay for this item with eBay Mobile because this item is contained in a multi item order.....please pay for this order on the eBay website....

***OK** (button)

Basically the APP will not allow you to pay on combined invoices, and it more or less forces buyers to pay on each item as they buy...