How do you power EU items? and how to ship an item to USA?

The EU has 230V mains. However, the wall-warts supplied with small devices often work 110-230V -- why not just ask the manufacturer?

I submitted questions on their official website, but no replies.

May I suggest you just order one? I use two, somewhat more upmarket, and the effect is excellent! The wall-warts supplied work anywhere in the world

I know some people use step down transformers.

Only thing you might need is a different plug or adapter. The provided AC/DC converter looks like it would work workd wide. It only needs 7.5 volts. Look through the photos of it on the site you provided it shows the plug. 

talkskiwon, there are two types of USB transmission to DAC - synchronous and asynchronous. The first one relies on computer to create timing while asynchronous USB sends data in packets and DAC recreates timing with its own clock. Most of the newer USB DACs use asynchronous scheme since computers cannot create reliable jitter free timing. Chances are that you have newer asynchronous USB and this device won't do anything - just wasted money (check it). Using good USB cable with data lines only (no power) might help. Device that filters out high frequency from the data line might help, but this device offers reclocking with "ultra precise clock" that most likely you don't need.  What is your DAC?

kijanki Thanks for giving me great insight on technical stuffs. I don't have the DAC yet, and I am getting a new integrated amp for my room,  which is a full digital, no DA converter, but tuned digitally to make class A amp sound. Reminded me of Denon 2500ne sound signature, but with much better in the lower end, maintaining same level of clarity and fun sensation in higher pitch. Just released in S.Korea a month ago. Their ad say they put a lot of effort in lowering background noise, but not sure what they exactly mean or how they did it. S. Korean companies are making all digitall all-in-one amps thesedays, like the Waversa Slim Lite selling on Audiogon. Not the same company of my new amp though.

talkskiwon, I would get the rest of the gear first.  There are basically two camps - accurate presentation camp and pleasant coloration camp (warm sound etc).  I belong to the first camp with Benchmark DAC3 HGC and Benchmark AHB2 power amp.   Benchmark DAC3 has very strong jitter suppression  for S/Pdif inputs but any electrical noise injected by physical connection can convert into unstable clock inside.  Because of that I decided to go Wi-Fi to Airport Express and then I use Toslink to DAC3, to avoid electrical connection.  

(I wouldn't buy anything from company that doesn't respond to emails)

kijanki I get a lot of recommendation for Benchmark DACs for the same reason you are in for those camps. My taste is slightly lean toward the colorzation camp, which a low physical intervention is much enouraged nevertheless. I understand you logics behind. Clear cut simple explanations. Thanks!