As I understand it, your contract is to call Vessel Assist (Towboat US), not Seatow. Some people actually have contracts with both companies because they worry about having just your situation occur.
However, if you didn't call and have Towboat US arrange the tow, I don't understand how they would have an obligation. Perhaps they do, but you should talk with BoatUS about that matter. It's sort of like having a service contract with Sears for your home's appliances and then getting upset when the Maytag dealer you called to service your washer charged you too much money. Sears would have little say in such a matter of how much money Maytag charges for repairs.
Your boat's hull insurance will typically deal with a salvage contract such as this. I assume that you don't have such an insurance company in play and if so, I suggest you get in touch with a maritime lawyer. Only such a person will be able to assist you properly. Each case is different and dependent upon the risk to the party who towed you as well as the risk to your vessel.
Questions that arise include:
How was the weather, winds, tide, prognosis for things getting worse?
Was your boat in danger of a hard grounding or worse if immediate action was not taken?
Was the incident inside a harbor?
What was the danger to the vessel assisting you?
Why did you decide to sign the agreement if your boat was not in danger? A soft grounding is just that--something that you, your anchor, and your dingy (and perhaps another cruiser) could have kedged free or just waited for a tide, etc.
If you have hull insurance--contact the company. If you only carry liability insurance--contact them, but then be prepared to call a maritime lawyer. Perhaps your liability insurance company can point you in the direction of a good lawyer.