Great question — answering as the UphillTech team:
It's pretty difficult to answer with any degree of accuracy, and it seems to defy logic, but the Service fees list tells us that, for a 20 kg bag, in low season, the price range is: Online: €20.00 – €112.50. At the airport: €70.00 – €80.00. While in your case you are probably in high season (the range is €25.00 – €122.00 when buying online, but no price is given for at the airport in that case), the price quoted seems to be much lower than the lowest price at the airport. Why, when and how it can become (much) more expensive online than at the airport is a mystery to me.
Full answer by jcaron: https://travel.stackexchange.com/a/203987 (CC BY-SA 4.0).
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