Regarding your basic question, the formalities of FAR Part 15 pertaining to discussions and clarifications do not apply to Part 13 procurements, so I see no reason not to contact the offeror for clarification about a quote. I don't believe before/after the RFQ closing date changes that, as long as the impartiality considerations at FAR 13.106-2(a) are followed and there is nothing in the basic contract to indicate otherwise. As a side note, when using FAR Subpart 16.5, other parts of the FAR such as Parts 13 and/or 15 are relevant too. In this case, you would need to follow the competitive requirements at FAR 13.106-1 if planning to make an award for an item that was not competed.