Store the customer name and discount rate in columns P and Q of teh Customers tab ensuring that the name matches that of the customers in the system in column B.
In the Invoice tab, cell F22 (the first item discount rate) add the formula: =IFERROR(VLOOKUP(custname,Customers!$P$2:$Q$500,2,FALSE),0)
This formula will replicate for subsequent items and import the discount rate if available for each customer.