Section 25 Valuation

Any  amount  supplier  is  liable  to  pay  relating  to supply ,    however  same  has been  incurred   by the  recipient   and not included  in the  Invoice  by the supplier  , must  be included  in  Invoice and charge  GST   as per  applicable  rate . 

Suppose Mr. A(Recipient) purchases goods from Mr.B(Supplier) and Mr.B hires services of Mr.C to transport the goods to the premises of Mr.A. Here Mr.B is liable to pay the transportation charges to Mr.C, however Mr.A pays the amount. Now if the transportation charges is not included in the invoice then for the purpose of valuation such amount should be included.

Yes it's simple any expense incurred by Recepient on behalf of Supplier for any particular supply , the Supplier is bound include the same when it's supplied to Recipient. Means he need to charge GST on Supply made + Expenses Incurred by Recepeint.