I did just this recently,
Send your paypal custom field to data as your would, inside that custom field, use a separator to split your data. In the below example, the values are split using a "|", you can use any character you want available in your charset.
$member_id = 1; $some_other_id = 2; <input type="hidden" name="custom" value="<?php echo $member_id.'|'.$some_other_id ?>"/>
This will output:
<input type="hidden" name="custom" value="1|2"/>
When you get the information from paypal (the IPN response) process it like so:
$ids = explode('|', $_POST['custom']); // Split up our string by '|' // Now $ids is an array containing your 2 values in the order you put them. $member_id = $ids; // Our member id was the first value in the hidden custom field $some_other_ud = $ids; // some_other_id was the second value in our string.
So basically, we send a string with a custom delimiter that we choose to paypal, paypal will return it to us in the IPN response. We then need to split it up (using the explode() function) and then do what you would like with it.
When you get your value from the database your select it using normal methods, then just decrement it by 1 using:
$val_from_db--; // Thats it!, Takes the current number, and minus 1 from it.