Seems like I've written this somewhere else, but anyway....
I don't think Jack expected to die at age 39. But he did have a certain sort of idea of early death lingering around his brain. Hell, if nothing else, Ennis had told him that they were both at risk of death if anyone found out. (And Jack never told Ennis that Aguirre had indeed seen them, and thus they had in a sense "cheated death" [and especially Jack, since he went back and Aguirre seemed somehow to blame Jack for the situation]--wasn't there a version of the script that had some guys threatening Jack after he leaves Aguirre's trailer in '64?)
Anyway, I don't think it's odd for married couples to mention to each other some thoughts about final arrangements. Perhaps Jack and Lureen were at the funeral of one of Lureen's relatives, or they passed a cemetery, or heard about some famous person whose ashes were scattered...somewhere. Jack certainly didn't want to be stuck in the Newsome section of the cemetery, so he told Lureen his wishes.
My brother and his wife are in their forties, and they have already made burial arrangements.
What everyone else said about the ashes going to BBM sounds right. I'll just add that, practically speaking, Ennis Del Mar was the only one who was going to make that trip and accomplish that duty.
So Jack was ensuring (or so he thought--vainly, as it turned out) one last connection with Ennis.
Okay, I think I'll cry myself to sleep now.