You can get a one time refund for up to 3 purchases (skins/champions), for obvious reasons I don't think they will refund any boosts. Yes, this is a ONE TIME thing, if you have 3 items you want to get refunds for, you need to request a refund for all 3 of them at the same time.
In order to do this you can simply go to here;
https://support.leagueoflegends.com/...s_requests/new
Under Category select Store Refund and fill in the blanks.
As stated in their knowledge base, "Any exceptions to this are handled on a case by case basis and are at Riot’s sole discretion."
https://support.leagueoflegends.com/...-refund-policy
Most regrettable purchase for me would be Jax lol.
Nasus, Sion and Tryndamere.
I suck with Nasus and Sion and Tryndamere is just outright boring, just prevent yourself dying and you carried the game...
I got a Corki skin, hot rod I think. I got a refund for it in the end, but it was a pretty meh skin, no idea why I thought it was a good idea to buy it.
Not even that good at Corki.
The only one i really regret is Evelyn, but i also dont play much Fiddlesticks, Riven, and Fiora seems a bit too squishy, so i might regret buying them a bit. But i'm sure i'll get better at them and will feel better about buying them. I dont regret them, but not as happy as with champions like Gangplank, Nocturne, Graves, who i love with the bottom of my heart.