Spirit Airlines' $100 carry-on bag fee is coming to passengers who wait until the last minute.

Spirit Airlines has discussed implementing a $100 carry-on fee for several months,but to clear up some confusion, not everyone will have to pay $100. Spirit Airlines is a low-cost carrier, meaning their fares are affordable. To make up for this, they charge high fees. Fees make up about 40 percent of the airline's revenue. On average, a passenger winds up paying $100 in fees for a round trip.

Starting November 6, Spirit will start charging a $100 carry-on fee, but this will only apply to passengers who declare their carry-on at the boarding gate. The airline has a tiered system for charging for carry-ons. Those that wait the longest get hit with the highest fees.

Spirit passengers who are members of the $9 Fare Club are charged $25 for a carry-on bag. Regular passengers who book and pay for their carry-on online will only pay $35. Those who do it over the phone will pay $40. Those who declare their carry-on at the ticket counter pay $50 but if they wait until they get to the gate, they'll get hit with the $100 carry-on fee.

While a $100 carry-on fee sounds outrageous, this is being done so that passengers won't wait until the last minute. When a passenger waits until they're at the gate to purchase a carry-on, the boarding process is supposedly slowed down for all customers  and can lead to flight delays which increases costs for the company, resulting in higher airfares for all passengers.

Instead of punishing all passengers who make arrangements for a carry-on in advance, only those that wait until the last minute are required to pay the highest fee.

"Our goal is for no customer ever to pay the $100 fee,"  Spirit's Chief Operating Officer Tony Lefebvre told Nasdaq.  "Spirit offers our customers multiple opportunities to avoid this unnecessary fee and save money. By planning ahead and paying for bags before getting to the boarding gate, our customers are saving time at the airport and speeding up the boarding process. When our customers choose these time-saving, self-service options, our costs go down, and we can pass those savings along to our customers." 

Checking luggage is cheaper than bringing a carry-on on Spirit. Starting Nov. 6, a checked bag will cost as low as $20 for members. Regular passengers who reserve a checked bag online will pay $30. Those who do it by phone pay $35. Those who wait until they're at the airport ticket kiosk pay $45 and those who wait until they're at the gate pay $100, which is the same rate for a carry-on.

Spirit's carry-on program has already proven successful. Many passengers decide to check their luggage instead of bringing a carry-on and many already pay for a carry-on in advance. This has led to a quicker boarding process, sufficient overhead space for all carry-on luggage and it has even helped the airline save up to six million gallons of fuel, which allows them to keep their fares low.

The airline allows everyone to bring a personal item that can fit under the seat free of charge.