I ALWAYS use flat rate AND go to the USPS.com to pay, print & schedule
for a pickup.
You can save money by doing it online. A flat rate padded envelope only
costs me $5.10 to mail if I do it online, if I take it to the post office to have
mailed it will cost $5.30. Now twenty cents may not sound like much of a
savings, but it all adds up.
I LOVE the pickup service the most. I don't have to go to PO anymore and
stand in line waiting, even though my PO is fairly small they never seem to
have enough people working the counter (usually only 1 person).
I HIGHLY recommend setting up a USPS.com account.
Just my opinion though.