The integration with PaySimple is a bit tricky as they do not allow you to add any custom code to their "thank you" page, but there is an option to add a button to take customer to a custom URL, which we use in this integration.
To start with the integration, login to your PaySimple admin account and navigate to Account Settings - Settings> web payment pages ... Here you can create or edit an existing payment form.
The tricky part begins here. There is a label field for your return URL and a field for the URL itself. You have to name it the way users really click it, e.g. "CLICK HERE TO FINALIZE YOUR ORDER" or something like that... If they do not click it, it won't be tracked unfortunately.
The return page URL should be a unique URL with hardcoded sale tracking code, with all params like total cost, product ID etc..
You can also set the URL to be one general URL with GET params specifying the product of this form, e.g.:
http://yoursite.com/orders/thanks.php?total=100&product=bicycle200 ... depending on which form you are modifying.
Do not forget to integrate your site with the click tracking code.