There are a couple of ways:
Create a text attribute for transaction items: https://support.woopos.com/knowledge-base/transaction-attributes/
Create a NOTE SKU with blank description: https://support.woopos.com/forums/topic/create-a-service-sku/
Add a note (internal or invoice note) at bottom of the payment window. To change the note, just double click the note in the F3 transaction window.