PayPal sandbox return URL being ignored despite option activated in profile

Question

• I've set up an order page with Add to Cart buttons and a standard View Cart function.
• I've set up a buyer and seller account in sandbox.
• I've tested the payment process and all works fine.
• I then activated the Return URL option in my seller's profile payment options and entered the URL that I want to send the user to (which should then process the PDT and give the user some feedback).
• I've of course entered the token generated after setting up Return URL in my script.

However, once a transaction is done I get first a confirmation page from PayPal with a manual "go back to seller's page" link. And it then automatically redirects me to the sandbox home page (logged out). This also happens when clicking on the manual link to go back to the seller's page.

What am I not getting? I appreciate any help on this. Lost so much time with that already...

Thanks, Sam

edit: adding these two lines to my form actually solved my problem:

<input type="hidden" name="return" value="your return url">
<input type="hidden" name="cancel_return" value="your cancel url">