Paypal IPN Please Help!

I need to add a Generic Tracking Pixel on the paypal confirmation page on my site. i believe it is
http://mysite.com/paypal/ipn

how do i go about placing this?

When placing this pixel into the sale confirmation / online invoice page, simply alter the following items.

- XXX needs replaced with the actual variable that contains the sub/total amount.
- YYY needs replaced with the actual variable that contains the order number.

Do i modify the paypal.module file? If so where and how do i enter the above code?

Thanks in advance for any help!