I can't figure out how to add shipping costs for my customers. I have an invoice set up, and everything looks great. But in some cases, we drop-ship direct from the manufacturer, so we won't be using FedEx, UPS, etc. for shipping. In other cases, we're shipping very large, heavy items, and we use shipping services other than UPS/FedEx. So, we need to put the shipping cost onto the invoice, but we can't figure out how to do it. Can someone PLEASE help???