[TYPO3-shop] Problem with mail and finalize

[Gueho]

When my payment gateway goes back to typo3 to finalize the order, sometimes 
an email is sent, sometimes not.

I'd like to check what is going on in the PHP but I dont' know where to 
search.
Could you please tell me what are the files used (and in which order) when 
an order is finalized.

A sample callback url called by my payment gateway : 
index.php?id=63&products_cmd=accept&products_finalize=1&FE_SESSION_KEY=4c3e964839-2ab1ca3aaa103a31de2f9fc671bf4e74

The id=63 page is the confirmation page. There is only a "plugin : PRODUCTS" 
on this page.

Thanks.

Fred