questionsis there an online, discount at&t retailer that…

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Try Walmart, maybe Bestbuy

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I haven't gone all the way through checkout and all, but it looks like Walmart will let you add a line and then share data on AT&T from their website.

Galaxy S III is $139.99, but it includes a $100 Walmart eGift card.

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@chris12345: Wow ... Simplexity (again, the man behind the curtain at the Walmart front end (who also runs Wirefly)) makes it really hard to get the right plan selected due to lots of back-and-forth resets of choices, but it looks like a pretty good deal with it all worked out finally.

I'll hold off a bit longer to see if any other deals surface, but this one is damn good.

Thank you, sincerely.

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If anyone's interested in what I've found so far in my further digging ....

Amazon sales reps have confirmed there's no way to order the shared data plans as a line conversion from them right now.

If you want to do it via Walmart, it's possible, but it needs some serious finagling with how the website is structured.

Here's the recipe for that:

1) Start at http://wireless.walmart.com
2) Select "AT&T" from featured carriers on the left (yes, this is obvious, but you have to choose it in the right place or this breaks I found.)
3) Hit "Shop Now" for the phone you want to add. In my case, this is a Galaxy S III in red.
4) Click "Add More Phones" button at the bottom left below the phone.
4b) Wait to update the selected second phone for a moment ... it's easier that way.
5) On the upper right, under "ALREADY HAVE AN AT&T PLAN" click "Add More Lines to My Account"
6) Select "I want to convert my individual account to a family plan"

CONTINUED BELOW

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How to get the AT&T family shared data plans to show up at Walmart's wireless website when converting from a single line - CONTINUED:

7) Choose "Smartphone"
8) Hit "Continue"
9) Click the "Change Device" link in the top of the "DEVICE 3" header and update it to the right phone. If you want to zonk it and go back to two lines, that's possible now, or adding additional ones too.
10) Above the orange "Add to Cart" button on the right, click "Change This Plan"
11) Click the "Mobile Share Plans" tab, which should be second-from-the-left.
12) Choose the desired plan
13) Do the usual checkout dance from here out... you should be good.

.... And that is how you get to a family-shared-data plan on AT&T on a single line conversion on the Walmart wireless website. Saner approaches leave you stuck without the option of choosing the shared data plans, I know not why.