When pricing a job for a potential client, and once all the details have been discussed it is important to produce an estimate to give the client. This gives the client an average for the cost of the service they will receive for the product or services required.
A purchase order is an order number for a product or service that is to be carried out, this gives the company (photographer) insurance that even if the client pulls out the photographer will not lose out financially.
The invoice will be issued to the client once the job is completed. This is a bill for the cost of the job, product or service.