Hi, a customer who successully bid on two items of mine is getting an error message when she tries to pay my invoice. "One or more items can not be paid for."

I've tried changing the shipping in the invoice to reflect combined purchase discount and also tried putting the difference in as a discount. The shipping is domestic interstate in Australia and I haven't change the type of postage from when the items were both listed. I haven't encountered this problem before, is it at my end or theirs? Is there a way around this?