I have been buying a few books from ebay about the size of a manual.

I have been noticing most sellers have been using those yellow envelopes with a few stamps on them and dumping them into the post box.

I guess that's the way they do it to avoid the lady at the counter.  

so keep doing it the old way and use stamps  

If buyers want the security offer them a more expensive option other wise it falls on them if the item gets lost.