Are these orders froma web site? or catalog? if web site then it is the responsiblitly of the vendor to update and put a sold out or back order notation on the item - if by catalog then again it's up to the vendor to communicate with the customer give them the option this way they can tell you if one item is more important then the other. Long story made short communication is very important to make sure that you have returning customers. I would tell the customer that shipping costs will need to be applied to the back order maybe consider giving them a break everyone loves a break -