Only unused in-game content and unused VP purchased within the last 14 days are eligible for a refund. In-game content counts as used once it's taken into a game: that means you've reached the loading screen of a match in any mode (including custom games or practice) with that content equipped.
^ use that link, log into your account tell them "Hey, I´m 15vp short off neptun spectre. Could you add some to my account?" and send them the artwork you did too