I've purchased many lenses from outside of europe, including from Japan and typically with no problems.
As already mentioned the import fees can be complex depending on a number of factors but I calculate for an additional 25% on top of what I've paid the seller…..That is usually fairly accurate, if anything more often than not the actual amount is a tad less.
Almost everytime that I've had items delivered from outside the EU, the customs fees have been requested at time of delivery, be it by the courrier ( Fedex etc) or on collection at the .P.O. and have had to be paid before the parcel has been "released".
On one occasion however the parcel was delivered and the bill for the customs fees arrived a few weeks later.
No big deal one would have thought, but having duly paid the amount, I received continual demands from the courrier company for the non payment of imports charges.
It took quite some time and a number of letters to resolve.
Don't let this put you off as some very good bargains can be had from Japan and elsewhere, just be aware of some of the potential hiccups that may arise.
If you're ready for such eventualities then they don't come as a surprise and as such can often be resolved fairly easily.