Cart wont work with more than 6 items

Is anyone else having this issue? When I add more than 6 items to the cart I get this error message "We can't process your payment. Please select a different payment method or check your payment details."

 

If it bring the number of items back to six I can checkout fine. Doesn't matter which items it is, it's if it's more than 6 items it plays up. I've tried several accounts and several different browsers/devices, and it makes no difference.

 

Ebay's solution was to pay individually, but I don't want to waste all my 5% vouchers.