A Progress Invoice refers to a document that offers a client to pay for the products and services in installments. Often used for long-term projects with large estimates and eventual budgets, this instrument can be the perfect option for customers that cannot or choose not to fund the project before all the work is carried out which offers flexibility and an opportunity to modify the budget over the course of the project. An invoice of this kind is typically sent to the client several times as a reminder of the percentage of the job that is already completed with a request to pay for it.