GST on service

As your query sounds that you are not a GTA . You are arranging transportation services to your customer.

Now , if You are charging only commission amount for the services being provided to your customer , then yes it's 18% GST on the same & bill for transportation goods by road is Exempted Under Notification 12/2017 CT (r) (But you are exclusively using vehicle for only single consignee under contract hence not Exempted)

The truck owner who is raising bill to you is GTA ,then you are liable to do RCM on the same.

Note :
In term of Section 15 (2)(b) "any amount charged for anything done by the supplier in respect to supply services is taxable"

So It's recommended that you shall raise single bill for Hiring Charges ( commission + transportation charges) & need to charge 18% on the whole bill. (Refer Notification 11/2017 CT r , entry no. 10)