Alternative to summing nested If's

I have a table of approx 20k customers (column A) and their $ sales (column B)for a time period. I need to sum the sales of approx 50 customers. I have the names of the 50 customers to be summed in one column so i can reference them and easily change out a customer if need be. Can anyone suggest an alternative to creating a nested sum that repeats 50 times? I can not change the source data at all, so I have to pull the data into anther area. Thanks.