The Flat Rate envelopes are $4.95. There are two different Flat Rate Boxes (FRBs) that ship for $10.35 (one box is 11" x 8.5" x 5.5" and the other is more of a "shirt box" style). If you use the Flat Rate APO boxes (12" x 12" x 5.5") the cost $11.95 to mail but are APO ONLY. That's the set price per box, no matter what it weighs up to 70 pounds.
When you order at www.usps.com, all boxes are free, as are the custom forms, and will be delivered to your house.
I personally prefer the FR APO boxes. For $1.60 more you can stuff a bunch more in there! The gel insoles will fit diagonally and it you are mailing writing paper you don't have to bend it in the package.
Just make sure to use the "good tape" and be generous with it. (you can also get free "priority mail" printed tape free from the post office, but its not that great IMHO) I've had boxes returned with can food add dented, but the boxes and seams held up well using the good tape.
I'm sure you have already read to never, never, never ship hygiene items and food in the same package. The cookies will taste like soap on the other end no matter how well they were originally packaged. Double bag any liquids. I even single bag any gels / creams / meltable solids (deodorant and air fresheners). If the box is received crushed the semi-liquids would (hopefully) not have effected the other items.
You will soon learn to fill every crevice with goodies for your contact. I have unboxes instant oatmeal and granola bars, flattened the boxes and shoved them between cans and sides of the box (I think we all do) so the contact understand that I'm not sending leftovers from my pantry. If sending food FRBs, use gum, small packages of nuts, hard candy or sucker as filler between items. If sending a hygiene FRB, consider using hair ties, pens, chapstick, eye drops, envelopes, sample items, etc. as filler.
Good luck and don't hesitate to ask for assitance/tips. We Junkies stick together! And always remember that someone on the other end of your shipment is thrilled to receive your packages.