The idea presumes a value of the CE at the game's release but I think what will most likely happen is that there won't be any physical boxed versions of PE at release(regular or collector's) and this will end up a backer exclusive item. therefore there won't be anything for the customs officials to google. If customs officials can't determine the value, they ask you to send proof of purchase (sales receipt, paypal transactions etc). In the netherlands the burden of proof lies with the recipient. If they see a collectors' edition computer game they will probably dertermine its price higher than € 22, digital deliveriy nothwithstanding; it also depends on accompanying paperwork and even transporters; fedex automatically sends almost every delivery through customs. Keep in mind, this is the Netherlands; I can imagine different practices for different countries, even if they are part of EU.
With regard to Taxes and Customs: I specifically ordered the Digital only tier, instead of Collectors Box, even though I'd love a cloth map. I live in The Netherlands, but this probably applies to most EU-countries. If you order physical goods above € 22 you will be taxed for customs, administration costs AND VAT. This is normally in the range of € 10 administration and handling + 15-30% customs and 21% VAT over total value (price of game+shipping costs+ administration+custom costs). To be short, it can become pretty expensive. So if Obsidian includes documents that the game costs only € 20,-- I'd probably be safe. But if a customs official does 1 google search regarding price of collectors edition of Project Eternity you'll probably get a pretty hefty tax/customs fine.