A few clarifications and notes:
1) When I say, "You can put anything in these," of course I mean anything that is allowed by the post office. It has to be legal is the point.
2) Domestic flat rate, unlimited weight, $4.95 before adding Delivery Confirmation, insurance, etc.
3) International flat rate, 4 lb weight limit, $12.95 before adding tracking, insurance, etc. Customs form must be attached.
4) Flap must seal on its own. Envelope must not be modified - cut, 2 taped together, etc.
5) Click here to order the Flat Rate cardboard mailer online.
6) Join my FaceBook Group!
Forget going to the post office - try Stamps.com. The primary benefit to Stamps.com is that it is the only online shipping site that allows you to print First Class International at home. I ship 75+ packages a week, 90% of them go out of the country as FCI or in Flat Rate envelopes. I never go to the post office!