If you're using the product-specific Payment Variable Information URL, then only the key issued for that product would be sent to that URL.
E.g., here you have [item_cart_position] => 2, so that indicates this particular submission was POSTed for [item_number2] => 799226, so that tells you which product the [key] => 546 was issued for. If the other product in the order also issued a key, that key would be disclosed in a separate submission to the Payment Variable Information URL you'd configured for that other product, so you'd want to look for another submission where [item_cart_position] => 1 for the same [txn_id] => jg-l8jedkb730f3a53, and get the [key] value from that submission.