I did the same thing just yesterday with Urf Rider Corki. I put in a ticket to see if they could reverse my rental. No dice.
I think at the very least Riot needs to consider making it more CLEAR (Isn't clarity one of the big design goals these days?) that "rental" means you DESTROY the shard in order to rent the skin for 7 days. I am a smart person with pretty good reading comprehension... nothing in the current UI made it OBVIOUS that the rental removes your ability to unlock the skin with essence after the rental is up. Renting gets a one line mention at best in the current Hextech Crafting FAQ... there is no explanation of the permanence of that choice in the FAQ either.
To me, the way the rental feature currently works is counter-intuitive and makes no sense from a "value" perspective. Why would you want to rent a skin for a week, when instead you could use it to unlock something permanent that you actually want?
I think the assumption that the rental is "temporary" and you get the shard back at the end of the period is far more prevalent than Riot expected/anticipated... but definitely something they should address... either by changing "rental" to what is intuitive, or updating the Hextech UI to make the function more clear.
Personally, I am heartbroken that I "threw away" Urf Rider Corki. I honestly thought I could just try it out this week, and would still have the option to craft it into permanent once I rounded up the remaining 200 Essence I needed. :(
Much regret. Much sadness.
Rito, please keep other people from knowing this sadness.