“I think halacha allows that as long as you are aware of the pricing..”
Obviously, if you signed a fixed price contract for X dollars, and the caterer arbitrarily tried to add a $400 surcharge after the fact, you could just ignore such a request. However, its obvious from your question you were aware of the “premium” in advance and in such cases, either halacha or civil law, its your choice whether to pay up or go find another caterer. Again, a caterer, simcha hall, Badchen, etc. can charge whatever he or she wants and there is no halacha that dictates pricing for services offered for sale. The price is whatever the parties to a transaction agree to.