Question: I recently bought two one-way tickets from Madrid to Cancun, Mexico, through Cheaptickets.com. I found tickets that were within my budget and called the online travel agency to verify all the details, including the baggage fees.

A company representative assured me that the fees listed on the Cheaptickets website were accurate, and that I would have to pay a reasonable 60 Euros per bag. So I booked the tickets on Air Europa.

But when I arrived at the airport, I discovered the flight was not operated by Air Europa but by Iberworld Airlines — and its baggage fees were excessive, to say the least. It charged 10 Euros per kilo. We had two extra bags to check, weighing roughly 50 kilos together. I was charged 500 Euros for both bags.

I contacted Cheaptickets and told them about the misinformation, and they told me the flight was not supposed to be a codeshare flight and that I was due a refund. However, I just received an email and Cheaptickets now claims because Air Europa will not return its calls or emails, there is nothing to be done except offer me a $100 travel voucher. Can you help? — Carla Stewart, Washington

Answer: Cheaptickets should have been able to give you reliable information about baggage fees and the airline you were flying.

Codesharing, which is airline industry-speak for allowing two airlines to share aircraft, passengers and other resources, is a practice I’ve long criticized for its dishonesty. But the least you can expect when two airlines engage in a codesharing relationship is that they will honor each others’ agreements with passengers.

It appears Air Europa and Iberworld didn’t do that.

Who’s responsible? Well, I think you did your due diligence by phoning your online agency. When an Iberworld representative insisted you should pay a confiscatory 10 Euros per kilo for your checked bag, you might have put up more of a fight. Asking for a manager or calling your travel agent from the airport could have resulted in Iberworld reversing its decision on the spot, although I can’t be sure of that.

It’s a good thing you used an intermediary to book your tickets, because it can and should act as your advocate when you’re overbilled by 500 Euros. Cheaptickets’ $100 certificate is a good start, but I’m not happy with its excuse that Air Europa won’t return its calls.

So what? Cheaptickets still gave you assurances that you’d be flying on Air Europa and paying 60 Euros per bag — both of which turned out to be untrue. In other words, it didn’t sell you the product you were promised. (In fairness to Cheaptickets, codeshare agreements and luggage fees can change by the minute, so it might not have known.)

Still, your agent should assume the responsibility for your flight problems. Otherwise, why bother using an online agency? Why not just buy the ticket directly through Air Europa the next time?

I contacted Cheaptickets on your behalf. It refunded your entire luggage fee.