What is the difference between Proforma Invoice and Commercial Invoice?

Proforma Invoice is not a real invoice, it is simply a confirmation of the purchase order before shipment of goods. Commercial invoice is what the vendor bills you after the goods have been delivered. Hence proforma invoice is not recorded as a liability on the books while the commercial invoice is.

Many times, before establishing a credit relationship with the vendor, the vendor will present a Proforma invoice to request a payment in advance (PIA) before shipping the order.