Flat rate is the way to go! I always ship Flat Rate Boxes. If it's just a little bit, I'll put it in a Flat Rate envelope, but I'll also have a regular manila envelope with me. I always ask for the difference in postage. They postman will tell you, just gotta ask.
If you use a Flat Rate box....cram as much as you can in there. They can hold up to 20 pounds. I keep saying I"m going to put stuff in a plastic bag, then suck all the air out of it, then see how much room is left. If there's more room, will try the plastic bag again and suck air out of that until I can get a full box. Then hurry and tape the box down before the air seeps back into it and explodes the box. lol