Estimate: an educated guess
An estimate is your best read on what a job will cost before the details are settled. It suits work where the scope is still moving or the price rides on things you cannot see yet. A contractor estimates a repair before opening the wall; a consultant estimates before the brief is final.
Say what the estimate assumes, and what would push the number up. An estimate that hides its own uncertainty is worse than no estimate.
Quote: a price you will stand behind
A quote is firmer. It names a price for a defined scope, usually with a date it expires. A quote is not automatically a binding contract, but once the customer accepts it, they expect that price to hold for the work it covers.
Send one when the scope is clear enough that you are willing to commit to the figure.
Invoice: the bill
An invoice asks for payment. It carries a number, an issue date, the line items, the amount due and your payment terms, and it becomes part of the financial record of the job. You might send it up front, at a milestone, on delivery or afterwards. What makes it an invoice is that it is a demand for money, not a renamed quote.
Where a proforma fits
A proforma invoice looks like an invoice but comes before the real one. It is useful for showing expected charges, requesting an advance, or providing figures for shipping and customs. Label it clearly as a proforma so nobody treats it as the final bill or pays it twice.
Which one to send
- The scope is still taking shape: an estimate.
- The scope is set and you are naming a price: a quote.
- You are billing for the work: an invoice.
- You need invoice-style figures before the real invoice: a proforma.
An accepted estimate or quote does not have to be retyped. When the work is done, reissue the same details as an invoice with a fresh number and a due date.