Charter prices are often quoted before tax, which makes them hard to compare. On ACE every option shows one all-in total, so the number you compare is the number you pay.
For flights within the US, the total adds the 7.5% federal excise tax and a small federal segment fee for each passenger on each flight. For flights leaving the US, your broker quotes the taxes, because they depend on where you're going.
Extras you add after booking, like catering or a later departure, can change the final total. Your broker confirms it with you before the trip closes.