Punjab State Power Corporation [PSPCL] — amount deduction was rs.100 greater than actual bill amount

Address: 144201

This incident has happened second time. When we open the website of pspcl, a pop-up window shows up which clearly states that online bill payment via debit card is free of cost.
Even during payment, the bill amount shows to be the exact same value that is entered by me.
But while i was at payment gateway, the amount was increased by rs. 100 approximately.
Also, the bill receipt doesn't show any description of the extra amount added to the original value.

Why is this happening?