I havent been playing long on this server, but from my previous server, they did this thing wherein certain costumes that come from rare things like an insanely difficult quests, rare drop from mobs or limited time things, after a while they put it on cash shop which invalidates the time or zeny investments that other players did to get them before.
I noticed that there are things here of similar nature, costume reward boxes, rotational slots prizes, limited cash shop boxes, seasonal quest events etc.
My suggestion is to not add the costumes that belong to these categories to cash shop just for quick buck. specially those that comes from the special sunday boxes and the slots.
I know costume is just costume, but idk, maybe there are others who are like me, who wears this costume as prestige items and work hard to get them and will feel bad if all of a sudden they become available in cash shop making the rewards from previous events lose their value.
Like for example, aura of ascension, it was available in slots before and is expensive right now because of exlusivity. If i farm for one, and all of a sudden it becomes available in cash shop, then it will totally invalidate my effort.
Hopefully, the limited costumes, or box exclusive costumes remain as that, to preserve its value and prestige.