Under 20cm thick you can send as a letter, if it is cheap or easily replaced then you just need the stamp, if you want to qualify for Paypal seller protection you will have to register, still available for letters.

Your PO should be able to provide you with a cardboard letter guide which you can use to check.

For anything that won't post as a letter then you are best off using click & send with the ebay branded satchels, they cost a few cents for the satchels and then $6.95 and $11.55 for the 500g and 3kg satchels. If an item is expensive then I add signature on delivery which is an extra $2.95 and for anything over $100 I add extra cover.