I lived in Poland when I ordered things from Amazon UK. I don't think that anything actually went through customs, because we are all in the EU. Certainly the packages were never opened and we were never asked to pay duty (as happened when things arrived from the US).
Personally I would not be comfortable only getting 3.6 mcg of B12 each day (or twice a day). It may well be enough but I wouldn't like to risk it.
But it is in toothpaste. And usually people have to wash their teeth every day.
And also it could be an easy manner to give it to small child.
And for the risk, i have asked exactly that. If there is a risk i dont take that.
But i dont think that there is risk. It a manner as another.
However i would like to listen some opinion.