When I am selling multiple items, I send  an invoice when we hit just over 3 weeks (minus postage), then keep adding the items to the pile.

Reason?  eBay start 'dropping items off' when they reach 30 days, and you can't combine the invoice anymore .... I had a great one last month, when a buyer went on holidays and forgot (legitimately .. she's a very honest buyer).   We ended adding items to the postage, etc ... sorted it in the end though.:)

That's it !!  We can sort it out between us, but eBay can't help EVER !!

I have also found buyers are unable to pay on a mobile devise (as another suggested) if they try to pay for more than one item.

AND YES .... I've learned my lesson  .... after being on the phone to them for nearly an hour this morning ... I resolved nothing,..  then went to the Discussion Page and solved it in a couple of minutes ....

You're a great bunch