A few months ago, Amazon's book-printing subsidiary CreateSpace ("CS") launched a new program that allows publishers to approve a book for distribution, without actually seeing a proof.
If I am reasonably sure that a book will be good enough to not embarrass me, I do not order a printed proof (which usually costs $15-$20 and takes a few days) but I tell CS that it's OK to distribute the book. In less than hour the book will be available for sale at Amazon.com.
With my money-saving system, I can order a book for second-day delivery with free shipping because I am a a member of "Amazon Prime." Alternatively, I can pay $2.99 for next-day shipping -- even for delivery on Saturday morning! Although I pay the retail price for the book, my cost is reduced by the publisher's profit I make on the book, and my purchase helps the book's sales ranking. Because I make a lot of purchases with Amazon/Chase credit cards (I have one, my wife has one, and my business has one), I accumulate lots of Amazon $25 gift certificates. If I use a certificate to buy my own book, and then earn the publisher's profit, the book costs me less than nothing. If I find major errors in the book I get from Amazon, I submit a revised file to CS, and the book quickly becomes unavailable on Amazon. This keeps people from buying a less-than-perfect book, but the book is still on Amazon and people can see it and order it for future delivery.
Two days ago, printer/distribution behemoth Lightning Source began offering "E-Proofs" -- a similar service which can save $70 or more and several days. (LS charges $30 to print and ship a paperback, $35 for a hardcover, plus at least $40 for an interior revision and $40 for a cover revision.) Full info on E-Proofs is available HERE and HERE.
The E-Proof is a PDF file, and should be identical to the PDF you submit to LS. The value of the E-Proof doesn't come from viewing the PDF, but in telling LS that it's OK to print and distribute the book. An E-Proof is not a substitute for a "P-proof" and is probably inappropriate for the first proof of a book, but can save money on subsequent revisions, and get your book on sale sooner.