In the magnified view of the stamp image, there are 6 more stripes visible, before the image is clipped off. The small area located in the lower-left corner of the design is clearly white. By counting the stripes below the blue canton, starting with the white stripe (number 8) it is very evident that there is a 14th stripe and it is white.
This raises an even further design issue ... if the flag was bounded by red stripes as the top-most and bottom-most stripes, which every flag issued by the United States since the founding of the country in 1776 has been, it would leave one to think that the flag on the stamp, if it followed the usual red stripe convention, would actually have had 15 stripes!
If the Postal Service is forced to change the design, these stamps may become collectors items. Link -via the Presurfer