When you use a padded envelope, how big can the envelope be to cost the normal rate? (6 forever stamps?) I've been trying to figure this out from the USPS website, but I'm having trouble finding information on dimensions.

Pretty sure it goes just by weight on 1st class parcel as long as it's under 13oz, above that it's a Priority category and gets charged more depending on distance.

$2.67 for any parcel "not over 4 oz".

Oh good. I was worried that a 9x12 envelope would be too big.