A proforma invoice is a document that you usually give to a client before you sell them the service or product.
If you’re self-employed and are discussing with a new client about a new piece of work, you might not always know how much you are going to charge them at the end. A proforma invoice is a temporary document that you can give them so they have some idea of how much you’ll charge them.
For example, let’s say you’re a freelance photographer. A client is asking you to help them shoot some photos of their office, but you don’t know how long that will take. You can give them a proforma invoice for, say, a day’s work. Then they can get an idea of what to expect.
You should also keep in mind that a proforma invoice is not binding – you still need to give them a proper invoice at the end.