Can we decide to award only one CLIN? Or, must we amend the current solicitation to award less than the full requirement?
It’s the classic case of “it depends”. It depends on what you reserved the right to do in your solicitation.
As a reminder, in general, FAR 8.404(a) says that the procedures of FAR parts 13, 14, and 15 do not apply to orders placed against FSS.
Based on information provided, I am going to assume this action is above the SAT and it was services requiring a statement of work. In that case the guidance is found at FAR 4.805-2(d) and it provides agencies wide latitude in how they structure their evaluation procedures and award decisions. It does not prohibit you from structuring your solicitation to reserve the right to award less than all CLINS. If you reserved the right to award less than all CLINs in your solicitation posted to GSA eBuy, then you are free to do so. If not, then it would be a very good idea to amend the solicitation and allow all offerors to revise their quotes (based on the rationale pricing decisions may change). Note: the DFARS does have some implemental language on purchases against FSS, but nothing that address your question or scenario.
Many organizations use the commercial contract format (SF 1449) when ordering off of a FSS. In this case you may have some more latitude if you included FAR 52.212-2. You will notice paragraph (c) includes the following:
(c) A written notice of award or acceptance of an offer, mailed or otherwise furnished to the successful offeror within the time for acceptance specified in the offer, shall result in a binding contract without further action by either party. Before the offer’s specified expiration time, the Government may accept an offer (or part of an offer), whether or not there are negotiations after its receipt, unless a written notice of withdrawal is received before award. (note bold italics added for emphasis).