Gazza, I sold two items. One could be shipped first class mail because it was under 13 oz. The other could only be shipped parcel or priority because it was over 1 pound. When I tried to make a combined invoice, it was impossible because it still gave the buyer the option of first class mail [which was no longer an option with the weight now being over 13 oz].
I sent my own invoice to buyer with correct shipping options. Just thought I'd let you know.