You could use 1 of 2 ways to get information back to your return URL, without having to use IPN or an API. You could use PDT to return information back to your site, or you could use the return method which uses the variable "rm". You can find more out about IPN here, and there are also some sample scripts you can use on that site as well.
If you want to use the return method, this is a little different from PDT. With the return method you do not have to post the information back to PayPal to verify it. If you wanted PayPal to post the information over to your return page you would just simply enable auto return in your account under your profile. Then you would just include the following lines of code in your button code.
<input type="hidden" name="rm" value="2">
<input type="hidden" name="return" value="http://www.mysite.com/return.php">
The variable "rm", controls how you want the information sent back to you. You can set it to be a GET or a POST.
Allowable values are:
0 – all shopping cart payments use the GET method
1 – the buyer's browser is redirected to the return URL by using the GET method, but no payment variables are included
2 – the buyer's browser is redirected to the return URL by using the POST method, and all payment variables are included
The default is 0.
Note: The rm variable takes effect only if the return variable is set.