The credibility of Booking. Com

I just made a booking with Booking.com. The initial booking indicated a total which I was happy to pay because it was a genius discount and further discount you would get if its a Pay now. affordable to me and after confirming my card details and paying, a new increased price came up. Why didnt Booking . com be upfront about the hidden cost that kicks in once you have paid. This is very very disappointing and ********* practice. Its really put a damper on me using booking com.