The Post also has priority "flat rate" mailers-$4.05- no weight limit on these eather. On the priority flat rate mailers or priority flat rate boxes- 2 different sizes on the boxes-. BE SURE you get the flat rate item(s). That says FLAT RATE on them. Or you will probally be charged the normal priority rate.-- This has happened to me.

Also on the priority flat rate mailer. If you are think of adding extra tape to the mailer. I suggest that you ask the postal clerk if it is OK to do so, before you do it. Because extra tape can possibally void the flat rate low charge.--This also has happened to me.

Any amount of tape should be ok on the priority flat rate boxed though. But note the priority flat rate boxes IS subject to the normal postal weight limits for anything of 50 lbs.

The above information is according to my local post office.
Dan