PayPal is easier than ClickBank, but both are possible with some custom script which takes some programming knowhow.
The way I do it with ListMail and ClickBank (the only way I figured it could/can be secured) is by storing the
cbreceipt,
cbpop, AND
seed in a MySQL database table. If you connect to my thank you page with a cbpop and seed that are valid and they are not found in the database you can create your member account (and run custom operations such as add to ListMail and send welcome message). If the member account has been created already (the verified vars are found in the database) you cannot.
With PayPal I like to use the IPN route, where PayPal notifies a specific script URL upon successful purchase. The script can verify the sale price, etc. With a simple invisible "open" to the signup.php URL (with the customer variables) adding ListMail to an IPN script can be done in as few as one or two lines!
All of this is quite involved and would take about 20 minutes to explain and 1-2 hours to accomplish (mostly due to ClickBank). Do you have programming experience or will you be 'outsourcing' this job? You may want to give
aMember a look, but I am not sure if they (still do not) support welcome/confirmation messages.
AShop is another paid membership management system.
Regards