Session expires after returning from pay pal to My site after payment.

Hi all,
I am Integrating pay pal to my site and using Pay pal PDT.
i AM USING HTML BUTTON OF PAY PAL AND i AM SETTING SEESION[uSERNAME"] IN my pay pal button page. from there it goes to pay pal and returns back  to my thank you page.
On my thank you page I am reading this session.
Sometimes its there and sometimes it null.
Some times Session expires after returning from pay pal to My site after payment.
P-lease let me know the reason if you can guess
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You need to add optional parameter(username in your case) to PayPal request and PayPal returns it in response ...
About session : You can change session time (20 min by default) in web.config
dhiraj79Author Commented:
Thanks for the reply ....
How can I send value of user name from my session variable to the html button code optional parameter.
Can you please give that code.
In your form you probably have a return URL like this:
<INPUT TYPE="hidden" NAME="return" value="">

Add the parameter to that:
<INPUT TYPE="hidden" NAME="return" value="<%=SESSION["uSERNAME"]%>>

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