I know that when you go into the post office and send a package under the priority shipping option, they will supply you with a box in which to ship the item, free of charge. I want to print the shipping label online, as the cost is considerably less expensive, but does this mean that I cannot obtain my free packing supplies, even if I take it into the post office? Ideally, I would like to be able to print the postage, take my item into the post office, request a priority shipping box, pack it then use my pre-purchased shipping label on the box that they provide. Is this possible?