Well, here are a few thoughts.
First, back then and possibly even still in rural America, you didn't have to know an address.
You could just put a name and the town and the correspondence would be delivered.
Second. My explanation for the envelope is not so much that is what Jack sent to him. I feel like Ennis probably scribbled the address on somthing else. He didn't want to draw attention from Alma as to how excited he was. Also as paranoid as he was he probably didn't want to run the risk of anyone seeing the postcard from Jack. No one woul dhave thought anything of it. However, I'm sure he was thinking everyone would be able to know from that.
Just my thoughts. No proof as to the envelope thing. It's just that I was Ennis for many years and I have a good idea of what his thought process might be!