PayPal works for me. You can build very much automated payment system around it:

- online user clicks on Buy link on your web page and is forwarded to PayPal front page

- user completes the payment

- once payment completed and verified, PayPal sends the payment information to your script (PHP, ASP etc.); at the same time it sends an email confirmation to your email address

Options and services are really plentiful, if I may say so.

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I also use similar ShareIt! and RegSoft services. Mostly to extend payment options. They are not bad; still PayPal looks better in every aspect, from my point of view.