I agree - time to think about your purchase. Additonally, you could introduce a special class of items that are *only* for the Achievement category. Now what that would entail (better, weaker, more unique) I`m not sure, but such a distinction might give developers flexibility in crafting items tailored for persistence in a campaign setting.
I had actually assumed that Persistent Item purchases would be acquired in-between skirmish actions, in some sort of character upgrade/selection screen.