Invoice: why make an invoice?
Customs Regulations (19 CFR 141.81) state that a commercial invoice shall presented for each shipment of merchandise at the time the entry summary is field. In order for the broker to complete or prepare your entry you must present him with a invoice. Customs does allow for a pro-forma type invoice to be used in lieu of an actual commercial invoice (bill of sale).
Even if you’re attempting to make an informal entry direct with U.S. Customs (not utilizing your broker) you must always have some invoice to present to the CBP Officer. Failing to provide CBP with some form of invoice removes a valuable tool they would need to do their job.
Try to remember that you’ll always need some invoice for CBP (Customs) when you ship goods to the USA.