Look at click and send. The satchels are 15c each if bought in 20s plus postage of $6.95. Far cheaper than the standard AP ones. And they are tracked to the door. The standard ones are only tracked to the delivery centre. If that centre is in a different postcode to the buyer, then you will not have Paypal seller protection.

As for what Tara said, I send roughly half of my items by untracked large letter (2cm max thickness). I have yet to have one not arrive.