When I was living overseas I tried shopping from Net-A-Porter and I found the prices to generally be a lot higher than if I purchased it in the US.  This is the case with most luxury items...including cosmetics.  
I believe Net-A-Porter now has a service center in the US, although, I don't know if this makes a difference pricewise.  
The tariffs are what make the prices so astronimical for purchasing designs from overseas vendors...also the availability.  A lot more products tends to be shipped to the US, as there's a lot more money to be spent here.  
I used to order my items in the US and simply have them sent to me in Europe.  I paid a lot less that way.  Also, Net-a-Porter is a british company and I find with the pound, the costs tend to be higher as well...it's not even so much about the exchange rate, as it is about "cost perception".  It's a difficult concept to get into here, though...  
Good luck with your purchase...