Flat rate boxes are usually the way to go if you have alot of things to stuff in a box. With the really small boxes starting at $4.95 and the largest at $14.95, it's really a good deal since you can stuff up to 70 pounds in the box.
Flat rate envelopes only take up to a certain amount of weight for $4.95 up to $12.95, depending on weight.
Out of country, what I do is....I ask how much the Flat Rate box will cost, how much a regular cardboard box the same size will cost and then the cost of the envelope. If I have to change what I'm mailing in, I do in order to not pay a ton of money. Sending to Canada and overseas can be costly, you just need to figure out the packaging.