Traditionally speaking, an Estimate will act as the “precursor” to an Invoice. You would normally create an Estimate first for a client, containing all of the items and services that you intended to charge them. Once the client has viewed the Estimate and approved the price, you would then “convert” the Estimate into an Invoice. Once it is an Invoice, the client will be able to make payment through the system, and any payments will be recorded and reflected on the Invoice in question.