On Amazon, if you pre-order something (including ebooks), you will eventually pay the very lowest price it ever was between the time you ordered it, and the time it ships. And your creditcard is only charged on the day it was ordered. On 06/17/2014 I pre-ordered JD Robb's Festive in Death at $5.99, it is currently at $11.99, and here is the fun part, I am actually getting it for $5.42 because sometime between when I ordered and now, the price must have dropped that low. So, from now on, if I am absolutely certain I will be purchasing a particular book, I will just pre-order it on amazon instead of comparison shopping.
And what this seems to work best on, is pre-ordering the ebook of something that is releasing in hardcover format.
Barnes & Noble doesn't have anything even close.
"If you pre-order an item and pay with a credit card, the authorization for that purchase usually expires in seven days for Visa, or 30 days for MasterCard, American Express, Discover, and JCB cards. For items that are due to arrive in more than 30 days, we may need to obtain another authorization from your credit card company. Your credit card is charged at the time we ship the product. The only exception to this is digital pre-orders such as NOOK Books, NOOK Magazines, NOOK Newspapers and NOOK Apps—for these items, the credit card account is charged immediately. "
So Barnes & Noble charges your book when your ebook when you order it, not when you receive it.
And I have absolutely no clue what Google's policy is on this. Either my google-fu is weak, or their policy is just no where to be found.