Running Totals in a particular cell
 

How do I get a cell to keep a running total? In other words, if I have a
list of customers, and throughout the day I need to keep a seperate running
total of each person's business, how do I add a value to an existing cell and
get the new updated total? Seems simple enough, but I can't seem to figure
out how.

EXAMPLE:

CUSTOMER A 50
CUSTOMER B 75
Let's say Cust. A does 30 more units. I want to be able to type in "30" to
Cust. A and have EXCEL show "80". How do I do it? thank you.