When creating a product you have option of adding a product code. The product code is only for internal reference and isn’t displayed anywhere on the customers invoices. This guidance advises on a workaround for this.
Adding to the Description
To show your product code on invoices, go to Invoices > Sales Codes > Select the Sales Code > Select the Product > Tick ‘Pre-fill the rate and description fields when I select this code. Using the description box, enter in your product code here, this description will be entered whenever this product is selected. When ready, click Update.