The flat rate envelope on the front says there's not limits, as long as you can seal with the original seal alone, you can put as much as you want in there. I had a postman tell me it would not get where it was going and would come back to me because it was not "flat". I showed him where it says that and he said "well, good luck...i think it will come back to you for more postage" - what a dip.