how to deduct tax & shipping to get sub-total ??

[JPRIETO]

Im using an affiliate program which is intergrated with prestashop 137

once a sale is made prestashop sends to the jrox affiliate program the TOTAL amount of the sale so that an affiliate can get commission

the problem is that this total contains Tax & Shipping amounts .... and the affiliate program calculates commissions based on these two amnounts also ..... not good

i just want prestashop to send the total amount MINUS the tax and shipping amount

the code i added to prestashop is this:

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$JAMIntegrate = file_get_contents("http://www.domain.com/affiliates/sale.php?amount=".$order->total_paid_real."&trans;_id=".$id_order."&custom;_mid=".$_COOKIE['jrox']);
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HOW CAN I MODIFY THIS CODE SO THAT ONLY THE SUB-TOTAL (BEFORE TAX AND SHIPPING) IS SENT BACK TO MY AFFILIATE PROGRAM?

PLEASE HELP!