Thanks! I bought one for my daughter when it was $50 off, ordered it eleven days ago, and it just came in two days ago. I contacted Amazon just now and got the additional $50 off. For anyone else who just bought one from Amazon on the $50 off sale, here are the steps I went through to quickly get the additional $50 refund (it's tough to find how to reach them by phone):
Log in to your account on amazon.com, go to the bottom of page and click on "help." On the page that opens up click on "Need More Help," then click on "Contact Us just to the right of that when the new page opens up.
That should bring up a page showing your Moto X purchase. Put an x in the box by that item, then in space 2 select issue, "Returns and refunds." another box appears for "Select Issue Details" and choose "other return or refund issues"
Then in #3, click on "Phone" and have them call you now. Tell them that you bought it on X date for $399 and it now is on sale for $349, and they will give you the credit. (at least, they gave it to me, no problem). It was a very quick process.