A traditional mail-based course offered by Esperanto-USA. You send in exercises, they correct them,
and you get a completion certificate at the end.
Easiest way: fill out the online form and ask to enroll in the free 10-lesson postal course.
Online Enrollment Form
You can also contact them directly:
Mailing address:
Esperanto-USA
2014 Capitol Ave, #100
Sacramento, CA 95811
Phone: (800-377-3726)
Email: eusa@esperanto-usa.org