It is not clear. If you did not agree to use them, why would they copy your material and do the research, and prepare an engagement letter? Did they tell you all those are free and included in $75 ? Next time, make it clear to your vendor how much you think you need to pay. Don't be afraid of talking of service fees.
For now, talk to them and negotiate a compromise. You are partially responsible for not being clear in your intentions.
Or you can refuse to pay. They might take you to court and you can go and argue. Your chance of winning is not zero, if you have the time and capability to represent yourself.