In order to answer the question, I would have to see your contract and how the items are priced.  I suspect this is an attempt to gain extra revenue.  If your solicitation was bid as fee for a service call and the item was priced as one price it would present that the “tuck fee” would be part of the supplier’s overheads.  To be charged separately it would have been a “separately priced item” and require a fee with a CLIN number.