I wanted to add that for any envelope/mailer you have to put a height for it in your package preferences.
Also CS does "not" read fractions under 1", so if you put .1" or .5" for an item height CS will figure at 1".
This is just on fractions under 1",
Over 1" it is not a problem, so you can put 1.5", 2.5" etc and CS will not round off to 2" or 3".
Also if you can get 3 items in one of these FR envelopes, then you need to put 3" for the height, if you can only get 2, put 2" or if you can get 4 put 4".
But if you can only get 1 item in the FR envelope, just put 1" for the height.
If you only put 1" for the envelope height then CS will NOT combine multiple items and will ONLY put 1 item in it and your buyer will be charged for multiple packages.
Also just because you have the Priority flat rate envelope in your package preferences does NOT mean that CS will figure this for an order.
CS figures the cheapest Priority method, whether regular Priority or the Flat rate option.
The type of Priority is "not" shown to the buyer. ONLY you will see the package type on your order page.