When is complete.tpl used?

[stormy]

I have a payment gateway script that provides its own "success" URL, but I might need to move away from it and use the regular WHMCS system where the ordering process ends at complete.tpl. So, I'm hoping to get some much needed info about it. There's almost no documentation and this is a crucial part of the ordering process. I need to do conversion tracking, upsells, etc.

-Is the URL always cart.php?a=complete - or does it have variations?

-Is it ONLY part of the cart process? i.e. for cart orders. If that's true, it will never get shown after a renewal invoice payment. Is that so?

-Does a customer ALWAYS end up at this page after a successful payment through a payment gateway? I count on the "ispaid" section to always work if the payment was successful.

-When/why does a customer arrive to that page without having paid? (so "ispaid" is not activated) Maybe if they enter wrong details on the payment processor?

-What happens with gateways that might take some time for the callback to arrive, like Paypal? Or Bank Transfer orders which are not real-time? I suppose ispaid will not be true so conversions won't be tracked.

Lots of questions, I know. I hope to get some answers! 🙂