RCM on transporter

With reference to the assessment of GST for FY 2017-18, the assessing officer is asking to pay 18% GST under RCM along with the interest on transport charges paid. (The said amount is borne by us and not charged to the client) Please confirm if it is 5% or any other rate at which it is payable.

If we pay 5% or so, can we claim it as GST credit?

Secondly, if we pay the RCM for the FY 2017-18 now, shall I get the credit of the same now or I need to write it off.