MIAMI — Frontier is set to become the next US-based airline to charge for carry-on bags. The airline, headquartered in Denver, will also begin charging for seat assignments. The changes, announced on Monday, take effect immediately on April 28th.

How much you’ll pay for carry-on bags depends on a lot on when you make the purchase: Part of Frontier’s [new] Discount Den? It will only set you back $20. Booking the ticket via the company website/reservations phone-line, or checking in online? $25. Prices go up steeply to $35 per bag if you wait until you get to the airport, and more than double the base price to $50 if you try to smuggle it through at the gate.

Seat selection, which includes middle seats, starts at $3 for regular economy seats booked online, though jumps to $8 once at the airport. Stretch seats, essentially Frontier’s premium economy product, start at $15 but depending on availability and length of flight, can climb up to $50. You can avoid this fee, however, by not choosing a seat, in which case the airline will assign one to you based on whatever is left.

Frontier already charges between $15 and $25 for the first checked bag. It also began charging for other once-free items, such as water and soda on board, last July.

Folks who purchase one of the airline’s Classic Plus fares can do either for free. Such tickets also receive one free checked bag, are fully refundable, and receive a complimentary upgrade to Stretch seats. The same is true of the company’s EarlyReturns loyalty program members who’ve reached its upper Ascent and Summit levels. Lower levels will be auto-enrolled in Discount Den, which, conveniently, requires a membership in EarlyReturns.

Frontier, which is in the process of being reformed into an ultra-low-cost-carrier, joins rivals Allegiant and Spirit airlines as the only three in the US charging for overhead storage space.

All three carriers depend on the extra revenue generated from such charges, known as ancillary revenue, to support their tantalizingly low base fares without going bankrupt in six months flat. If the other two that have gone ahead are any indicator, the change will certainly be good for Frontier’s bottom line. Allegiant took in $96 million in the first quarter of 2014 in ancillary revenue, representing nearly a third of its reported $302 million in revenue. Spirit, considered the ancillary revenue king, took in $668 million in the calendar year 2013.

The charges have hardly made the airlines in question popular, however.  Spirit outpaced the next closest US carrier threefold to win the highest complaint record in 2013, according to a study that analyzed Department of Transportation records. The airline is regularly lambasted for its fees and poor service.

Frontier came in a distant second place, though it has been rising steadily since 2012. Unsurprising news, since the carrier has gradually been shifting to a low-cost carrier since the turn of the decade. It began its shift into ultra-low cost territory late last year, when Indigo Partners LLC bought it for $145 million. Indigo is headed by the same exact folks who turned Spirit into the ancillary-revenue-monster it is today.

In fairness, many of those complaints are likely to stem from passengers who were unaware the charges existed to begin with. Items like carry-on bags and seat selection for normal economy seats have been free for so long they’ve practically become a bonafide right. And carriers that do charge for such things have faced accusations that they intentionally make it difficult to discern how much they’re paying, or if something is charged for at all. It doesn’t help that the system is set up to penalize, or profit from, depending on your stance, folks who show up to the gate with an undeclared carry-on – in some cases charging double the base price.

Perhaps seeing a rise in complaint rates, and/or a tanking of its popularity written on the walls, major carriers have yet to follow suit. Legacy airlines such as American and Delta are unlikely to do so for the foreseeable future, while low-cost carriers JetBlue and Southwest still don’t charge for first checked bags, let alone carry-ons. Time will tell, but it is fair to say that, so far, the allure of the almighty dollar has been difficult to pass up.