Contract Mistake

What is the name of your state? Maryland

I am a student who is a Graphic Designer and I was taking advantage of.
I had a verbal agreement with a small company to do a website. He paid me for just the initial fee but he did not pay me for the complete website.

The only thing I have is what he wrote for what we agreed on for price. Also I have an invoice that I gave to him that he signed and I signed saying he owes me the balance.

Do I have a case with these two evidence?

Verbal agreements mean nothing. Yes you have enough evidence to go into small claims court, but next time have a contract drawn up in advance with the total price of everything involved.