Best scenario how you can use estimation app for payment reminder too.
Let Customer is purchasing items from your shop. Now he is not paying at the moment.
After sometimes you or your person have to followup customer for payment. So before sending a person to collect payment if you just prepare estimate and share with customer, your customer has clear cut idea about what he has purchased from you and how much he have to pay. Also he/she can ready amount before your or your executive reach at the place. In caese customer has not yet ready to pay he/she can inform you in advance.
After payment you can generate invoice for the payments they have made. By sending invoice your customer have instant invoice and awesome experience deliver by you.
And make your month accounting clear for actual payments you have received.