In retrospect, I think you guys are right on about the postage. The size requirement for postcards is most likely just for the lower postal rate. I seem to remember sending a panoramic postcard about 10 years ago that required the standard stamp rate, which was much less then than it is now...
Also, thanks for the tip about RC holding up better. It is much more rigid and seems like it would last well. I have a small part of a 25 pack of Ilford Satin finish that I may try to use to avoid the more obvious handling damage that glossy finish apparently suffers.
Does anyone mount flush to the edges on cardstock and stamp the backs of their cards? I was considering mounting the prints to archival cardstock to stiffen them up a bit and to allow me to make a postcard-type design for the back.