Apparently you can buy postcard backing paper that you stick on the back of the 4x6 print. This gives you the same backing as on Ilford postcard paper and is probably easier to write on. It can of course be added to any paper of your choosing. It adds stiffness to it so is probably the equivalent in stiffness and weight to Ilford postcard which is Portfolio paper.
I don't know about prices in the U.S. but in the U.K. I can say that nowadays Ilford Postcard paper is a total rip-off( is this a U.S. term? If not I mean by "rip-off" an extortionate price) in terms of price.
About $70 per 100 sheet box. All Ilford paper has risen a lot in price in recent years but in about 2005-7 this paper was being sold at about $24 per 100 sheet box.