Paypal IPN process more than one custom variable
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So I've implemented paypal IPN in my site and I'm in the middle of the
work process. Now I want to use more than 1 custom variable in the pp form
currently I'm using this one only

    <input type="hidden" name="custom" value="<?php echo $user_id; ?>">

So I know the variable with the name 'custom' is allowed. I want to know if
I can pass more variables so I can filter the payments based on their criterias.
So if for example shipping is more than $0.00 I set a variable "shipping_cost" like this:

    <input type="hidden" name="shipping_cost" value="<?php echo $cost; ?>">

or for other purposes. Is this allowed? Or paypal has an already defined list of allowed
variables we can use? I really want to solve this problem as there's not always one type
of payment we can process...
Thank you guys.