Arithmetic

Let X = A + B

(A and B are positive numbers)

A company had quoted its client for X
The company bills its client for B first
and then bills the remaining balance of X (which means A) later

but the company sent the client another bill for X
(so client got charged for X + B)
client found out the error and explained to this company several times

hopefully the company can finally understand the problem
so this arithmetic episode can be closed