Shopify Purchase Order - How to record tester products

Hi All,

I have a question on Shopify Purchase order system. I need to record a purchase order for skincare products, within this order there are tester items. For examples, a lip balm testers is $3 and the whole sale price for the same item is $4. How to track the lip balm tester products within the Purchase order form so it accurately reflect the price I pay to the vendor?

As you can see below, Shopify only allows adding products from the inventory list. Any recommendations on how to keep track of this? Thanks