Item rates for individual customers

Item rates for individual customers

Hi - 
We only bill 3 customers using Zoho.  They are weekly customers but each customer pays a different rate for our services.  I am aware that the rate is editable.  My fear is that sometime I may forget to change the rate when billing a customer and charge the wrong price.  Is there a way to tie an item to a customer so that each item has the correct rate when a specific customer is selected?
Thank you for your time,
Darla