You could flash a generic firmware as close to the one you had originally. Best to ask in sub-forums for your particular device. Also, ask if it's possible to downgrade to your original firmware, if indeed you installed a higher rom.
You would have to hunt around for the flashing program, windows drivers & the full flash rom for your phone, & the instructions.
I would go ahead & root. You should only get charged for repairs if you damaged it. I took mine in & told them it was rooted, I had a common fault so they were happy to repair it & charge it to the manufacturer. I think you were extremely unlucky to be charged if it wasn't your fault, root or no root.