Enter deposits into estimates

It would be helpful to be able to add a deposit into an estimate. We use estimates from the time the customer contacts us (could be many months prior) until the day they actually received all their items. During those months, they edit the items, change the amount of items needed, add, subtract, etc. If there were an option to have the deposits included in an estimate, the customer could have a "running total" of what they have on order :-)