If Statements to calculate negative numbers

Hello Forum,

I'm hoping someone can help me out with what I believe is a simple formula that I cannot seem to get to work.

In column C is the amount that I will charge a customer.
In column D is the amount the customer paid me.
In column E is the balance owing.

A simple formula in cell E2 to figure out the balance owing is this:
(E1+C2-D2)

The issue I have is this :
If my customer pays me more than they need to (we work on large projects here and they like to "pre pay" ahead of time), I need it to add their payment to the balance.

I've attached an example.