Jon Elson writes:

An alternative is to spend a half hour reading the info on line
and then take the on-line test for the EPA "green" card that
allows you to buy R-12 in small cans for vehile A/C service.

While I would agree that this is worth doing, note that you have to own
several $1000s in tools (recovery systems, etc) to lawfully apply that
license to any actual work. Otherwise you're an EPA felon.