Little help with a paypal variable

Hello all,

I am well aware that this question could very well NOT belong here but I visited one of paypal's support forums and it made me wanna cry so I'll try my luck here.

I am setting up a payment system and want to automate it as much as possible so I'm setting up paypal's Instant Payment Notification (IPN), if I use the Instant Payment Notification (IPN) simulator and give it a specific value on the "txn_id" field I get what I want, which is to update the status of a transaction on my DB.

Now, if I place a test order on paypal's sandbox everything is fine but it seems that paypal is not receiving my txn_id value and is sending the IPN with another value.

I am sending the txn_id like so:

<input type="hidden" name="txn_id" value="<?php echo $txn_id ?>" />

$txn_id is being retrieved from $txn_id = mysql_insert_id(); (I'm inserting the data of the order prior to sending the client to paypal) just so I can update the status from pending to completed when I receive the IPN.

I guess the final question is why is Paypal replacing or not using the data I'm passing as txn_id?

Thanks for any help provided :)