A flight website offered me change/cancellation for a fee, now refuses and blames airline [closed]

I recently used a website (mytrip.com) to book a round-trip flight (with 1 stop in each direction). The website I used for booking is not the airline's own website. Now, I was not sure I would actually want the exact date I was booking, but I noticed I was being offered a 'premium package' for an extra fee, in which I could edit the flight details or cancel at no charge.

And, indeed, I ended up having to do that several days later; I needed to postpone my return by 2 or 3 days (the return flight is in two weeks from now). When I asked to do so, the website/company-behind-the-website told me it would not be possible, since the airlines does not permit changes or cancellations. I told them that they sold me that option, and they claimed I should have read the fine print. Actually that fine print was sent to me in Dutch, but even disregarding that fact, they can't sell you an "option to do X" and have the fine print say "you can't do X"... I would think.

So, my question is - is there anything I can do about it? I'm not averse to paying extra (obviously, not the price of a new ticket) to make a change, but nobody seems to be willing to allow me to make it.

Notes:

• I called the airline directly; they said that because my ticket is in a cheap fare category, they will absolutely under no circumstances allow any changes, even if it the flight I want was coming up and they had empty seats they weren't going to fill.
• If legal jurisdiction is an issue somehow, I live in the Netherlands and the flight I want to change is from TLV to AMS.
• If you need more information in order to be able to give an answer, ask.