In the USA I use ebay’s standard envelope when possible.  My minimum charge for this is $1.25.  Due to the restrictions on this service I may have to use First Class Package.  My minimum charge for this service is $3.50.  However, I do not charge for additional purchases that can ship in the same envelope.  For example, buy one item for $3.50, or buy 10 items for the same $3.50 total.
For items shipped out of the USA my minimum charge is $2.75.  Many small lots can usually be shipped with this $2.75.  Larger items or many items will require additional postage.  An item larger than  6″x9″ will have to be shipped at a package rate.  Registration is very expensive ($16 + postage).
I have been a member of the American Philatelic Society since 1977 and try to make my descriptions accurate.  Please let me know if I missed anything.