When the PayPal transaction is complete, PayPal shows this:
Thanks for your order
Your payment of $XXXX.XX USD is complete.
You\'re now going back to MYSITE.
What you ask is achievable with PayPal Payments Standard, but the seller account receiving the payment needs to be a PayPal Business/Premier account (a free upgrade), and they need to enable Auto Return and provide a Return URL in their PayPal account > Profile > Profile & Settings > My Selling Tools > Website Preferences.
You can also pass a return
URL variable to checkout for each order, which would override any Return URL (or lack thereof) in the seller's PayPal account profile; however, if Auto Return is not also enabled in the seller's account, the buyer would need to click past the end of checkout manually in order to be redirected to that URL.