Unless otherwise provided in the appropriation concerned or other law, purchases and contracts for supplies or services for the Government may be made or entered into only after advertising a sufficient time previously for proposals, except

(1)

when the amount involved in any one case does not exceed $25,000,

(2)

when the public exigencies require the immediate delivery of the articles or performance of the service,

(3)

when only one source of supply is available and the Government purchasing or contracting officer shall so certify, or

(4)

when the services are required to be performed by the contractor in person and are

(A)

of a technical and professional nature or

(B)

under Government supervision and paid for on a time basis. Except

(1)

as authorized by section 1638

(2)

when otherwise authorized by law, or

(3)

when the reasonable value involved in any one case does not exceed $500, sales and contracts of sale by the Government shall be governed by the requirements of this section for advertising.

In the case of wholly owned Government corporations, this section shall apply to their administrative transactions only