H. P. Gas — extra price I am paying when paid by credit card.

I booked a hp gas online on 05.11.2018. On 08.11.2018 at 7.53.55 pm i paid rs989.98 the cost through credit card having transaction no.[protected]. On 09.11.2018 the gas was delivered to me and i find that the billing is rs.972.00. Now i have paid rs. 17.98 extra while paying online when the present policy of govt. Is not to make transactions by cash. Had i paid by cash at the time of delivery i would have paid rs.17.98 less.
I want a clarification on this point early & demand the extra paid by me to be refunded.
Regards,
Mahesh chandra praharaj., cosumer no.620083,
Palaspalli, bhubaneswar, odisha.