Letter and Light Packet are the cheapest. Neither carries either tracking nor insurance so build in a few pennies for self-insurance against any “non-arrival “ claims.
Light Packet is more expensive, but when they are printed out from Paypal, you get a number. It’s just the CP invoice, but provide it to your buyer in feedback or email. That will protect you against some fake claims.
You do know that any payments will be held for up to 21 days against your good performance right?