Would they really charge 70 Euros for a slight discrepancy in paper size? If so, thanks for the heads-up!

If a US business were to attempt such exacting and draconian enforcement, the government would require prominent disclosure of such non-obvious requirements. (For most purposes A4 and letter can be used interchangeably.)

In the US, courts give a lot of leeway to customers for contracts of adhesion, where the customer is presented terms written by the business as "take it or leave it". No business operating in the US could charge disproportionate fees for insignificant departures from stated rules, especially if those rules were not explained fully and stressed heavily in the purchase process. Perhaps this is why RyanAir does not yet serve a US airport.