This is the title of a Google AdSense banner ad that keeps appearing on this forum (Some of the banner ads say "500 business cards for $8.50" ). I thought I'd tell my experience with this company: GotPrint.com
I've never tried their 500 postcards for $20, but I have done their 1,000 postcards for $59 several times. The quality has been top-notch and the turn around has only been a few days (average has been 6 days, including the shipping time).
The company offers 500 full color standard size (4x6 inches) postcards printed on heavy, 14-pt card stock, with a nice clear, protective UV coating. For your $20, you get an online proof, but no printing on the back side. If you want black printing on the backside, it's an additional $21.40 (for a total of $41.40)! This why I spend $18 more to get twice as many postcards, AND with black printing on the back side.
The $20 price is a great promotional deal if you just want full color on one side, with the back side blank.
Since I usually need 1,000 cards at a time, and with the back side printed black (for resale use at gift stores), I pay the $59 price. It cost another $11 to have these 1,000 cards shipped to me from their Burbank, CA plant (they also have plants in KY and TX) to my home in Salt Lake City, UT. My total cost is $70, or 7-cents each. I wholesale the cards to gift shops for 15-cents each, making a 100% mark-up. The stores resell them for 25-cents or 30-cents each.
If a card starts selling well, then I order 5,000 postcards at a time, and reduce my total cost per card (including shipping) to on 3.25 cents each, making a 200%+ mark-up!
I also give away my postcards to friends, family, and use them for promo pieces to potential clients (to entice them to buy large prints, because every card has a web address on the back).
The $20 for 500 postcard promo from GotPrint.com is a killer deal, even if it doesn't include black printing on the back. You can use these as giant business cards (like I do), and print out labels to go on the back if you need to.
I have no connection with Gotprint.com other than I do business with them, and find them to be a quality printer.
Warning: In order to place an online order, you should have a good working knowledge of Photoshop and be able to convert your sRGB files to CMYK (via Edit > Convert to Profile > "Working CMYK - U.S Web Coated (SWOP) v2" )). You'll also need a color-profiled monitor so that you'll have the confidence that the online proof you get from the printer is matching the color you see on your screen. If you don't understand what I've just said, you should hire a good graphic designer who does (or get yourself a Mac computer). ;-)