My reference to the small flat rate box is not about profit/loss but that it is EASY to ship and you can fill the form out online with USPS and be done with it. GSP is the way to go because the buyer pays all shipping fees and customs/import taxes up front, this USUALLY leads to a small increase for the buyer but I have been surprised to find that a few times, the shipping/import fee has been cheaper than I can offer on my own. If you use the GSP, then eBay takes care of customs for you and you limit your risk in the package getting lost or misplaced. There is an entire section on eBay about the GSP and I recommend that you read it as it is thorough.
There is something wonderful about selling something to Europe, Asia, or Australia and simply dropping it off, postage paid, with no waiting, no paperwork, and no worries.