130 to the property involved and other relevant considerations shall be given for a period of at least 15 days before making a purchase of or contract for property, or a service, under clause (7) or (8) of subsection (a) involving more than $10,000. (e) A report shall be made to Congress, on May 10 and November 19 of each year, of the purchases and contracts made under clausei? (11) and (16) of subsection (a) during the period since the date of the last report. The report shall— (1) name each contractor; (2) state the amount of each contract; and (3) describe, with consideration of the national security, the property and services covered by each contract. (f) For the purposes of the following laws, purchases or contracts negotiated under this section shall be treated as if they were made with formal advertising: (1) Sections 35-45 of title 41. (2) Sections 2T6a-276a-5 of title 40. (3) Sections 324 and 325a of title 40. § 2305. Formal advertisements for bids; time; opening; award; rejection (a) Whenever formal advertising is required under section 2304 of this title, the advertisement shall be made a sufficient time before the purchase or contract. The specifications and invitations for bids shall permit such free and full competition as is consistent with the procurement of the property and services needed by the agency concerned. (b) Bids shall be opened publicly at the time and place stated in the advertisement. Awards shall be made with reasonable promptness by giving written notice to the responsible bidder whose bid conforms to the invitation and will be the most advantageous to the United States, price and other factors considered. However, all bids may be rejected if the head of the agency determines that rejection is in the public interest. (c) If the head of the agency considers that any bid received after formal advertising evidences a violation of the antitrust laws, he shall refer the bid to the Attorney General for appropriate action. § 2306. Kinds of contracts (a) The cost-plus-a-percentage-of-cost system of contracting may not be used. Subject to this limitation and subject to subsections (b) - (e), the head of an agency may, in negotiating contracts under section 2304 of this title, make any kind of contract that he considers will promote the best interests of the United States. (b) Each contract negotiated under section 2304 of this title shall contain a warranty, determined to be suitable by the head of the agency, that the contractor has employed or retained no person or selling agency to solicit or obtain the contract under an understanding or agreement for a commission, percentage, brokerage, or contingent fee, except a bona fide employee or established commercial or selling agency maintained by him to obtain business. Tf a contractor breaks such a warranty the United States may annul the contract without liability or may deduct the commission, percentage, brokerage, or contingent fee from the contract price or consideration.