Most Chinese sellers undervalue the shipments. So, they probably wrote that the package is only worth $20. I doubt there will be any import tax.
I've ordered packages in bulk from China a few times and never got a fee. It came via DHL in about 2-3 days. Pretty fast shipping times.
Also for $290, why not just get the Nexus 4? Or did you want a 5.7" phone?
FYI, the specs on the page say it supports
Support 3G network: GSM 850/900/1800/1900 & WCDMA 850/2100 MHz
So, it only supports the 850 band on ATT, not the 1900 for 3G. Might be semi-congested on that band?