what you should pay

is the fair price of 2 hours' service. I don't know where you are. You can call some other firms to get quotes. If the quote is significantly lower than that you can call them and negotiate.
But just because there is no contract doesn't mean you get free service. Well, unless you are willing to take your chance to the court or deal with a collect agency.
BTW, the fact that you are not happy with the quality of the service is not a reason not to pay.