got the same thing whit the undefined thing, but it is on all the diapers, so i have no idea what any of them cost.
also the White, Blue and Pink Diaper seems to be More Expensive then the more Special/bigger capacity Diapers fore some reason, usually you would believe the specialized and less commonly used Diapers would be the Expensive once, not the Basic once.
so that's probably way it look like you are unable to buy them.
also some times the Diaper Sales Menu doesn't seem to load properly and not shoving what you should be able to buy from it.