It's a low-cost carrier, what do you expect? They only have interesting prices on flights, everything else is regular fare. And the best way is to ask all your questions to
frontier airlines customer service before buying a ticket. Also it is cheaper to pay for extra baggage when buying a ticket, because the highest price for a baggage item is at the airport. If you keep this in mind, it will be easier for you to pay for the extra cost. Have a great flight!