Actually, to be more precise, it doesn't matter what it weighs as long as it will fit into the Priority flat mailer provided by the post office.

The correction is:
....will fit into the Priority flat "rate" mailer....

There are 2 versions of the priority mail envelope. One says flat rate. The other does not. The flat rate envelope can be mailed for $3.85 regardless of weight. The mailing cost of the other envelope will be based on weight & zone.
The envelopes look the same except for the flat rate statement.