It all depends on how many bottles you want to send. If I send around 3 bombers, I'll use a flat rate but more than that I'll use my own. Actually I figure out it isn't that much difference in price to use your own box and have peace of mind instead of trying to squeeze so many bottles into that flat rate.
If you already have a box you can calculate the rate on the usps website to give you an idea of how painful its gonna be.
