How can I accept Paypal payments

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I am running vBulletin® Version 3.7.3 and would like to know if there is a program that can accept Paypal payments for membership without having to register first.

For example: A customer purchases the access to the forum form a HTML website (different domain name) and after the payment has been made the Paypal transfers the customer to the forum where he is instructed to register and once the registration is finished he gets the full access to the forum.

Current settings are:

Gests : can not see the forum is "PRIVATE"

Registered members: can see the forum but can not post

Paying members have full access ( payments thru the forum )

I can change this if necessary to accept payments from different domain names.

Thanks for any help.