You can't combine the invoices, and your buyer can't combine the payments, since they are in different currencies.
So you will be paying the non-refundable service fee of 30c on each payment.
But you can combine and ship the purchases together.
I actually had this happen this weekend. One sale on dotCA, two on dotCOM, and a delay in paying for one of the dotCOM pending the answer to a question. The books are going out in the same package to catch the 2 pm pickup.