So this is a long rant, but this tip saved me from getting jipped out of over a grand, so I figured I'd share. Of course, YMMV.
TLDR for the lazy: Expedia wanted to charge me an extra ~$1000 in fare differences plus $50 in fees to change just my return departure date (same airline, times, etc), but I ended up bypassing them altogether and getting it changed for free.
Story Time / Rant:
I found great prices (~$800) on round-trip tickets from EVA Airlines (US to Hong Kong with a long enough layover to go in and visit Taiwan as a bonus) and booked the travel through Expedia (after also checking Google Flights, Skyscanner, Kayak, etc.). From HK, I booked separate trips within Asia to other cities.
The trip started great with getting randomly upgraded seats - Yay, extra legroom and free alcohol! Once there, the food and experiences were great. I literally gained 5lbs in less than a month from all the desserts and excessive eating. How do the people there stay so skinny? And I didn't even get sick like I usually do while traveling through Asia.
Some friends that were also in the area at the time wanted me to stay an extra week to go on a tour with them, so I call Expedia (now about a week before my original scheduled departure) to find out the change rules on my tickets. They transfer me to a manager, who explains that my EVA tix arerefundable, full-fare, etc., and he assures me that I can change them anytime before the scheduled flight and just pay a $50 service fee (required by the airlines) plus any difference in ticket prices. I'm on the PC looking at new dates and ask if any of those have a price increase, and he checks and says no since they're all the same flight. I hadn't gotten exact details from my friends yet, so I ask him to make a note in my account and say that I'll call again to complete the transaction.
A few days later, I call Expedia back but get put on hold for 20 mins to talk to a rep (and this is supposedly the priority line for their preferred rewards members). I mention what the past manager had said and ask her to look into the file. She says she has to escalate to a manager anyway and I end up waiting on hold another 20 minutes. When the manager (in a heavy Indian accent) comes on, I explain it all again, but I get put on hold while he researches rates and the call gets cut off. Despite giving them a call-back number, I don't hear back from him. So I call again. Same deal, US low-level rep, transferred to Indian manager. Almost an hour into the call, cut off again. I'm starting to think they're hanging up on me on purpose to avoid putting in any effort.
All of these have been international calls, so I'm pretty pissed at this point. But I already made the rest of my plans based on the new departure date, so I don't really have a choice and call them again. This time I ask for escalation immediately... wait on hold... ask the manager to check on the tickets without putting me on hold... and we make it all the way through to new flight booking. Finally!
I pick the new date and she replies that she'll happily apply the remaining unused value (about $300) of my old ticket to the new ticket (~$1300), and all she needs is my credit card confirmation of ~$1000 plus the $50 change fee. I confirm this is the same exact flight that was in my notes, that somehow when from being free to costing more than the whole of the original multi-leg itinerary. I ask if it's because of the date, and to check another. She says if I want to change the ticket, the cost is mandated by the airline but Expedia doesn't have any additional change fees or costs. I passive aggressively tell her I could buy a whole new set of round-trip tickets for less than the price of this single leg and then I politely hang up.
Well crap, what a colossal waste of time! A friend suggests calling EVA directly to plead the case, but it's already after their office hours in the US and it goes to voicemail. (EVA, you really ought to consider forwarding the US main customer service number to one of your other offices to offer 24 hour coverage.) I find and call a local EVA ticketing office instead, and thankfully the rep spoke English, albeit with a heavy Chinese accent. I ask about changing the return date and she asks to put me on hold.
At this point, I'm jaded and thinking that I'm about to repeat the same process all over again. To my surprise, she's back on within a minute asking me which date I want to change the ticket to. 5 more minutes and she's done. I get to keep the same check-in/confirmation scan code as before, it has the same flight time, and best of all, I get to keep all my money. There was no extra difference in ticket costs, no $50 fee, nothing extra at all.
In the past month of being back, I've mentioned this story to several friends and family and it seems to be a pretty common occurrence. The third party reseller/agency sites quote exorbitant fees and costs while blaming it on the airlines, whereas the airline ends up often not charging fees or waiving them. One friend had a similar experience of nearly getting extorted out of $1500 for a new ticket, but she ended up getting a free date change directly through Korean Air instead. Another went to the nearest airport and was able to make changes for significantly less at the ticketing counter. So the ones that had the presence of mind to try a different channel of customer service either cut their costs or had it waived (granted this is anecdotal with a small sample size of 3 out of 3). Whereas the ones that took the charges as being unavoidable are kicking themselves now.
Moral of the story: Many [well-ranking international] airlines are likely more invested in keeping their passengers happy and coming back to them, so they have incentive to solve problems simply and in your favor. They also prefer having customers in the future book directly with them to avoid paying referrals, commissions, etc. So potentially they may force 3rd party sites to uphold certain charges and fees that they themselves ignore. Or perhaps the other sites are inept and haven't trained all their reps well enough to figure out how to avoid crazy extra costs. In either case, even if you booked your tickets through another site, it may still be worthwhile to try reaching out to the airline directly if the reseller's quote seem unreasonable. You never know if they're trying to get you to pay a lot more for something that's actually available for free/cheap.