Forum Moderators: buckworks
You can set this up using Auto Return in the Profile of your PayPal account, or you can just add a variable named "return" to your payment button and can set the value of that variable to the URL where you want the customers to end up after the payment.
To enable Auto Return, log in to your account, click on Profile, and then click on the Website Payment Preferences link in the column on the right. Set "Auto Return" to "On", fill in a default URL for your buyers to be directed to after a payment in the "Return URL" box, and scroll to the bottom of the page and click "Save".