Problem/ Question on accomplishing bank payment the right way

First of all, sorry for a long post and also sorry for my English, my mother tung is Swedish.

In the checkout process (asp, ecart 4) I gather the necessary information about the customer (name.asp). Then after hitting "Ok", on the next page (payment.asp) the user can, besides review the order, choose his/her own bank from the 4 bank-logos and upon clicking on one of these the user is taken to his/her bank to pay online for the purchase, these logos are submitting a form with a bunch of hidden fields. This is the only way for me to do it, it's by the bank's rules for online payment's. This means that this very page is the last page that I can influence in any way before the transaction is to be taken place (or rejected by the customer or the bank). This means that I have the order sent upon clicking one of these bank-logos. This is too early. It could be that the user chances his/her mind before making the transaction at the bank or maybe the transaction simply fails for other reasons.

I have tried to make this kind of work by sending another mail to the store owner when the customer is taken to the "thank you" -page after a successful payment. This mail includes some information of the payment embedded in the address bar which I'm able to pass on in the mail.
This is obvious not satisfactory as every order that comes in has to be checked against another mail.

Now the store owner insists that the order is sent only after a bank transfer has taken place.

I guess this would have to be done by NOT emptying the cart upon clicking on the bank logo on (payment.asp) and that the order would be sent at the "thank you" -page after a successful bank-transfer and then the cart would be emptied. But how? Is this possible? The payment is made outside of the site, isn't the session lost? I know I am ;)