You can go to USPS.com and order boxes and they come to your door...for FREE! :D I ship handmade soap a lot, and the small flat-rate envelopes are a lifesaver. It's usually $5 versus $10-$11 had I done it just regular priority mail. The post office ladies love to look to see how much I saved. (me too!)
And a word about first class: you can only ship first class if it's under 13 oz, so with THAT weight, it's better to go 1st class. International 1st class is up to 4 pounds, and I can't remember at what point it gets cheaper to do the priority flat-rate...