These days, getting a good deal on an airfare isn’t as simple as buying a cheap ticket. Unwitting customers may be lured in by amazingly low ticket prices online, but sometimes the deal ends there. With domestic carriers tacking on extra charges for all sorts of basic services, travelers are often blindsided by additional fees once they arrive at the airport. You may not be able to avoid every a la carte service you’re hit with. But knowing which airlines charge the most egregious fees can help you make a more informed decision when comparing fares and calculating total costs.

1. Carry-on fees: up to $100. If you’re hoping to dodge a checked bag fee by bringing a carry-on item onboard, think again. Three airlines (Allegiant, Frontier and Spirit) now charge for carry-on bags. The fees could increase significantly if you don’t follow protocol or you’re unaware. For example, the fee for a carry-on on Spirit will vary greatly depending on where and when you pay for it. When purchased during online booking, you’ll pay $35, and $45 during online check-in. If you hold off until you arrive at the airport, you will be charged $50 per carry-on. Wait until you get to the gate and you’ll pay a whopping $100. Similar terms and fees apply to checked bags.

Spirit Airlines is notorious for offering low fares, then tacking on a slew of exorbitant and hard-to-avoid fees. A quick search turns up dozens of articles like this one, which help customers outsmart the fee-hungry carrier and avoid extra charges. According to the Wall Street Journal, fees brought in two-fifths of Spirit’s revenue in 2013, so despite public annoyance, it has no incentive to eliminate or reduce them.

2. Selecting your own seat: up to $80. Choosing a specific seat on a plane is as easy as selecting an empty seat on a diagram with a single keystroke. Yet that doesn’t stop five major carriers from making passengers pay for this so-called privilege. Allegiant seems to be the biggest culprit when it comes to a seat selection fee, with customers complaining of charges as high as $80. In some cases, the cost of choosing a specific seat can be as high as the ticket itself. Allegiant states that when passengers check in for their flight, whether online or at the airport, they will automatically be assigned a seat at no cost. However, those traveling with a companion or in a group must pay extra in order to sit together. Imagine how quickly that adds up for families with small children.

3. Printing a boarding pass at the airport: up to $10. Last year, Allegiant implemented a $5 charge per boarding pass if it’s printed at the airport by a ticket agent. "We now have mobile scanning technology in even the smallest airports in our network so that every Allegiant customer can ‘go paperless’ and use their smartphone or tablet to check-in, pass through security and board their flight,” Andrew Levy, Allegiant Travel Co. president and COO, said in a statement. Spirit charges $10 to print each boarding pass at the airport.

There is no doubt that services like paperless check-in can save travelers time and money, providing they do their research and fully understand each carrier’s unique protocol. On major airlines like American and Delta, customers can print their boarding pass once they arrive at the airport without paying a fee. Check-in procedures, baggage policies and fees vary so greatly from one carrier to another, that travelers are often forced to scour the fine print or navigate a maze of red tape. If they don’t do their homework, they will likely get burned.

4. Booking a ticket by phone or in-person: up to $40. Another example of conflicting policies between carriers involves online versus in-person bookings. All domestic carriers, with the exception of Spirit and Frontier, allow users to purchase tickets online without any additional fees. If you opt for a person-to-person booking, either over the phone or at the airport, most airlines, with the exception of Frontier and Southwest Airlines, charge a fee ranging from $15 to $25. US Airways charges the highest price for tickets issued by phone or in-person: $30 for domestic travel and $40 for international trips.

Spirit charges a passenger usage fee, from $8.99 to $16.99 each way, when you book online. Allegiant tacks on a $10 convenience fee per flight for tickets purchased online. However, no fees are applied to tickets issued at the airport on either carrier. Both carriers have backward policies that are directly at odds with other airlines. According to Conde Nast Traveler, both Spirit and Allegiant’s rationale is strategically based on consumer behavior and convenience factors. “See, these two airlines know it's very unlikely that you'll actually go to the airport, and you'd be willing to pay more just so you don't have to do that,” author Brett Synder says.

5. Bringing a pet onboard: up to $250. Expect to pay a minimum of $75 if you are traveling with a pet. If you have a larger animal who needs to be placed in the cargo hold, you will likely pay a lot more. Fees for bigger animals are more justifiable since airport personnel is required to handle and transport them. If your pet is small enough to remain with you in the cabin, you’ll still pay through the nose to carry him or her onboard yourself. Keep in mind that pet fees are broken down by one-way fares, so if you’re taking a roundtrip flight, you will have to pay double. In some cases, a pet’s ticket could cost the same or even more than your own, and your pet doesn’t even get a seat.

When it comes to pet flight fees, United is the biggest offender, charging as much as $250 one way; Hawaiian Airlines trails close behind with fees up to $225. Why should it cost so much to travel with your pet? There’s no real formula here; it’s all about what the market will bear.

6. Change your ticket: up to $400. Airlines often delay or even cancel flights for various reasons, and offer little (if any) compensation to inconvenienced passengers. However, when travelers have to change or cancel their reservations, they are penalized with heavy fees. This is one area in which budget carriers Allegiant and Frontier seem to be more forgiving. Allegiant currently charges a non-refundable ticket change fee of $75 per segment. This same fee ranges from $50 to $100 on Frontier. American Airlines and Delta both charge a steep $200 change fee on domestic flights. Delta may charge up to $400 on international flight changes.

The list of ridiculous charges for some of the most basic things—beverages, inflight Wi-Fi or even a pillow—could go on and on. In an industry that is constantly changing, or as some believe, conspiring against its own customers, sometimes your only choice is to go without or suck it up. Even if you know the ins and out of domestic travel and have mastered the challenge of dodging extra fees, you also know all too well that the rules are subject to change at any time.