Scenario: I received quotes for a furniture buy (GSA) below the SAT, FOB Origin. Successful Contractor sends an original quote of less than $200K on SF1449 with no request for Advance Payment. During the technical evaluation, I submitted some questions to the contractor concerning colors, etc. Contractor clarifies the questions and submits a revised quote requesting 50% advance payment at the time of order. I send the awarded PO to the contractor, whom does not sign it stating that they requested, advance payment. Request that the contract be modified to reflect advance payment. He sends a third email stating that the awarded contract needs to amended for an additional $2000 because the prices increased. What's my best course of action?