What is GSA's policy regarding adding supplies/services to MAS delivery orders, where those supplies/services are not under MAS contract?

As stated in the ATA Defense Industries case of June 27, 1997 – GSA’s procedures satisfy the requirement of the Competition in Contracting Act of 1984 (CICA) since the Federal Supply Schedule (FSS) contract prices have been competitively awarded based on price negotiations and evaluations prior to award of the FSS contract. However, GSA has not negotiated or evaluated prices for products and services that are not listed in the FSS contract. Therefore, customers must purchase “incidental” open market items using appropriate competitive procedures. Customers should use competitive procedures or combinations of competitive procedures that are best suited under the circumstances of the procurement to assure they obtain the lowest overall aggregate price. (This decision is consistent with previous GAO decisions covering incidental items.) After a customer complies with requirement of full and open competition for the incidental item it may be placed on the delivery order for administrative convenience.