Hey professionals, is there any way to add "FBA fulfillment fee per unit" on item fulfillment record? Thanks in advance

yes, when you create a custom field you can have it apply to that record. However, a custom field wouldn't impact COGS. What is your end-goal?

Hey thank you so much,but I want to add shipping cost to the freight company.

The native way of handling this is to enable charging for shipping. Then setting up the charges on the shipping items. SuiteAnswer 9270 should get you started.