Is there any other online retailer besides amazon that will offer Wii U systems? The advantage of buying online is you don't have to pay sales tax, which is a considerable amount on something that costs $350.
"Don't have to pay sales tax" is generally relative to how much you care about following the law. Most states that charge a sales tax require you to pay sales tax on internet purchases if the retailer doesn't collect them at POS.
There was an article on another site about this a couple days ago.
If this is the same article I've found, then...
IGN poster Sarabellum1976-tk posted this in response to an Amazon thread regarding sales of the 3DS XL on the site.
...the "source" is a random posting on an internet forum.
I know how popular Amazon is but, I seriously doubt they buy as much Product from Nintendo as Walmart. We're talking the good and the bad.
Beyond that, I have serious doubts that if Nintendo even cuts Walmart some super-special deal that Amazon would even remotely know the details. You can be sure that if any such deal is in place, Nintendo isn't giving out that information (to prevent others from trying to get the same deal) and Walmart isn't giving out that information (because Walmart would never give up such information regarding their business advantages).*
*Disclaimer: Spoken as a member of the general public with some common sense, not as an employee of any business with super-top-secret information.