Auto Increment Specific Cell Value Based On Different But Related Cell Value

In my excel workbook, I have a customer table and invoice sheet among many others.

As part of my system, there is functionality to grant new customers with a discount on the first purchase. Within my customer table, the last two columns are "Number of Purchases" and "Customer Type" (either single/multiple depending on no. of purchases), which are then used to determine whether the discount is valid or not on the invoice.

Once an invoice has been created, archived and refreshed with the customer selected (via a Customer ID), I would like some code to auto increment the number of purchases on the Customer table for that specific customer to +1.

For example, customer called Bob (Bob-1);
Number of purchases = 1
Customer Type = Single
Discount = Yes

Invoice then created for Bob (using Bob-1 as the unique value), sent off and refreshed. New figures should be:

Number of purchases = 2
Customer Type = Multiple (can be achieved by using IF statement on No. of purchases)
Discount = No