Your invoice pretty much defines what you are claiming is owed to you, and why it is owed. It's pretty hard to add to it after it's presented.
You are saying that the work was agreed upon, and has been performed as promised.
Sometimes there are reasons - for example, your own internal accounting needs- to break the price apart.... for example, into sections for thise items subject to sales tax, and those that are not.
You might write the invoice in a manner that will make things easier for the customer to pay you (due to his own accounting / management quirks). For example, you might define the job as "periodic maintenance" or as "repairs," as the customer prefers.
The invoice is your opportunity to define other, additional, charges to your original quote. An example of this might be the rental of a lift or generator.... or additional charges caused by scheduling.
Finally, the invoice defines just what is covered by warranty- and when that warranty begins! A simple bill for "services rendered" is pretty much useless to everyone.
