No definite answer here; you would have to ask the people who did it.
So contact Now Broadband on 150 if you have a landline phone, or on one of the following numbers:-
0330 041 2518, 0330 041 2462, 0330 332 3050, 0800 759 1213, 0330 041 2473
My educated guess, though, is that you have asked to push the payment date later in time, and £15 is to cover broadband between your original payment date and your new payment date, on which the regular payment amounts will recommence.
Does that square with your current monthly payment and the number of days you have pushed it out by?