Then the only thing there that counts is the 4.99 Euros. The retail key can't be "transaction'd" by Valve because how can they know how much you paid for it? For all they know, you were given Portal 2 retail as a gift.
At worst, you'll need to get a Steam wallet/pre-paid code of $20 (whatever that is in Euros) and then redeem that to instantly get past the $5 fee. But convert the Euro's into dollars (https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=4.99 euros to us) and you get... 5.39 US Dollars currently as the trade-off, which SHOULD count.
LMAO. How the hell is this giving Valve more money? Anyone that is wanting to participate in the Steam community is more than likely already going to pay more than the $5 "fee" as you call it to buy games or items in CS:GO/et. al. already. Anyone that isn't can still play those free to play (TF2, DOTA) titles ONLINE. WITH NO FEE. The only thing they can't do is participate in the community and trade those items as protection against scammers.
So imagine a guy or girl who had been gifted games but never bought anything via steam. This person is an active user on the forums and has friends IRL who she wants to add on steam. She / he doesn't want to buy anything so has to add $5 to an account in order to use the features already used on a regular basis. This money then sits in valves account gaining interest until such time as it is used leaving valve with the interest.
Subscription fee is the wrong term, admission or membership fee maybe works better.