Yep, I had to pay more tax than I'm supposed to, though it would be the same for USD, albeit with less volatility.
Well, I'm not sure here now, but the law probably says that one of the rates (date of purchase, date of import, date of paying VAT) is applicable.
What I was wondering is, what if they asked me what the rate was?
Normally, the rate that is taken is the rate that is published by the central banks (in your case, the ECB), so there's no ambiguity. Since the ECB does not publish "official" exchange rate for Bitcoin, the one prevailing on markets should be used.
Another question is, if Google decided to support BTC conversions, which source would you suggest they use? Anyone can publish any price they want, and AFAIK no one is bound by law to be transparent about it (not that I think it's necessary, but that would be the first thing a legally responsible entity would ask). For instance, if I was gathering an average value from all exchanges weighted by volume, any one of them could artificially inflate its volume to affect the price. Am I thinking too much?
I would use the spot price from the exchanges. Since the volatility is so high, for the purposes of tax calculation probably precedents would have to be set.
Also, after having thought about it for a while, what you paid for probably was not an import levy, but VAT. Most goods imported into EU are not subject to import levies, but almost everything is subject to VAT.