Anyone who’s been caught caught in the middle seat on a packed economy flight can appreciate what Air Canada is proposing: The airline is rolling out an auction-style program that will allow passengers to make cash bids to upgrade to premium economy or business class.

Airline analyst Rick Erickson warned Air Canada is walking a fine line.

"The last thing you want to do is annoy those passengers [who paid full price]," he told CBC News. "You don't want to condition them into thinking that a cheaper price could be had."

Auctioning airline seats is not new. Priceline has made a business out of it. Virgin America has been particularly aggressive about it, working with PlusGrade and SeatBoost to manage the program, Skift reported.

PlusGrade works with more than 50 airlines, including Lufthansa, Qantas, Aeromexico, Air China, LatAm and Air Lingus. American gave it a whirl in 2013 but dropped it after merging with US Airways.

Air Canada said its upgrades will be available for select destinations. Passengers will need to visit a special webpage to check eligibility and make an offer. Or if a flight was booked through Air Canada, passengers will receive an email inviting them to make an offer 10 days before departure. If the offer is rejected, Air Canada will send a notification via email.

The offer page has a little slider to help passengers determine whether their offer will be taken seriously and Air Canada is offering “no guarantee” an offer will be accepted. Passengers will find out 48 hours before departure.

Travel blogger Anshul Singh tried it on a flight from Toronto to Dubai. He bid $500 for an upgrade to business class from premium economy and said though the webpage told him his offer was “weak,” he still was a “little disappointed” when it was rejected.

"There's no clear parameter put in front of you other than a little slider that tells you a good, bad or best [bid] based on Air Canada's presumption," he told CBC.

“Air Canada takes into account several criteria when assessing your upgrade offer. They include the amount of your offer, the amount of other offers, as well as the number of available seats,” the airline said, adding there will be no explanation if an offer is rejected — and there are no second chances or refunds.

The process will be available online only, meaning no phone calls.

"The program is designed to generate additional revenue by offering customers an opportunity to upgrade at the last minute," Air Canada spokesman Peter Fitzpatrick told CBC News in an email.