You program's documentation should tell users the registration amount
and how to send it to you.

Registrations can be sent in many forms: checks, money orders, and
credit cards debits.

Authors can also consider if a registration service, or subcontractor,
might work for them. The service usually takes a percentage of the
registration in trade for collecting the registration and keeping
records.

If you expect registrations from the international public, it is good to 
state the registration amount in U.S. dollars and/or to give the amount
in ways related to the countries were you expect registrations to come
from.

If you're interested in being able to accept credit card orders for 
registrations, you can try though a local bank, but may have better luck 
with CardService International at 1-800-456-5989.

I have not tried it, but a few electronic cash systems exist on the 
Internet. The most prominent is Digicash's e-cash (see 
http://www.digicash.com).

For shareware registration services, see the links listed at the end.