Submit multipage form to PAYBOX

Hello,


We have migrated our website to Joomla 2.5 recently with CF and Flexicontent. Before I had to forms one after the other.
The multipage option was chosen because I cannot use <FORM/> tags in CF.
I need to submit to a cgi file for PAYBOX.When I put this in General > Submit URL =cgi-bin/modulev2_CentOS_42.cgi, the multipage doesn't work (submits directly after Page 1) and when I use a redirect User action on Submit event, data is processed through the URL (GET) not with the POST method...

How can I do this ?

Page 1:
- people enter the invoice number + a random number (written on the invoice) + a valid email
Page 2 :
- a PHP script checks in a file if the numbers given exist and match together and brings back the amount to pay
- hidden fields needed by PAYBOX are filled with these data
- a submit button should submit to the cgi file (here's where I'm stuck)


This was the previous 2d FORM :

<FORM ACTION = "cgi-bin/modulev2_CentOS_42.cgi" METHOD = "POST" name="paybox" id="paybox">
<INPUT TYPE = "hidden" NAME = "PBX_CMD" value="SERFA_' . $numfac . '"> 
<INPUT TYPE = "hidden" NAME = "PBX_TOTAL" value="' . $montant . '"> 
<INPUT TYPE = "hidden" NAME = "PBX_PORTEUR" value="' . $email . '"> 
<INPUT TYPE = "hidden" NAME = "PBX_LANGUE" VALUE = "FRA"> 
<INPUT TYPE = "hidden" NAME = "PBX_MODE" VALUE = "13"> 
<INPUT TYPE = "hidden" NAME = "PBX_OPT" VALUE = "www.mysite.fr/cgi-bin/serfapaybox.txt"> 
<INPUT TYPE = "hidden" NAME = "PBX_EFFECTUE" VALUE = "http://www.mysite.fr/web/fr_merci.htm"> 
<INPUT TYPE = "hidden" NAME = "PBX_REFUSE" VALUE = "http://www.mysite.fr/web/fr_regret.htm"> 
<INPUT TYPE = "hidden" NAME = "PBX_ANNULE" VALUE = "http://www.mysite.fr/web/fr_annul.htm"></td></tr>';
echo '<input type="submit" name="btn_paybox" value="Payer ma facture"/></FORM>

Thanks for help