Printer specifications are real and accurate and meaningful, but are not to be confused with image resolution. Printer ink dots and image pixels are simply very different things, and one color image pixel requires many printer ink dots. This is why we need a 1200 dpi printer (ink dots) to print an image at 250 dpi (pixels).
Q: What are the standard postcard sizes you offer? A: For flat standard postcards, available sizes are as follows: 4 x 6, 4.25 x 6, 5 x 7, 5.5 x 8.5, 6 x 9, and 6 x 11. Folded postcards have 2 kinds: half-folds and tri-folds.
According to http://pe.usps.com/Archive/HTML/DMMA....htm#wp1002733 minimum thickness for a USPS postcard is 0.007 and maximum thickness is 0.016. Typical coated cover stock thickness is: 80 lb .008 100 lb .010 110 lb .011 120 lb .012 100 lb cover (270 gsm) is probably the most popular thickness for postcards.