Combining two orders for overseas

A customer ordered one item yesterday and then another today. I have to fill out custom forms as I'm in the UK and they are in the US. I had already printed out the label for the earlier order so how do I go about sorting this as the customs label is wrong now and can't be changed since I've bought the label and printed it. Should I just do two separate parcels and buy another label since they've paid postage twice? I was trying to be kind and was gonna refund them one lot of postage but I can't work out how to do that with the customs form problem.