There are a LOT of costume items available!
Some of them are credit only purchases, but most can be bought with event currency.
I recommend you farm the currency for costume skins on alts and use those alts to buy the skins. All wardrobe unlocks are shared across your region (NA or EU) so you can buy them with alts and it will unlock on all your characters in that region. You can even use alts on another shard to buy wardrobe, as long as they are the same region.

Some of these costumes will auto-consume when you buy them, meaning they do not appear as an item in your bags which you can then sell back to the vendor like you can in other areas of the rift store.
Some of the costumes are required for cheevos, you will receive the actual item for these, but you need to equip the item in your gear screen (not just your wardrobe screen) this will make the item bound to account so you still can’t sell it back, you can post it to alts so that all your chars can get the cheevos.

I recommend you WAIT until towards the end of the event before you buy your costume items, this is because you may get some of the older wardrobe items as random drops from minion adventures or from the Carnival Instant Adventures.

Note: The costume tab used to be split into proper categories, Helmets, Weapons, etc. This was broken in 2020 and everything is now just lumped together in one category.