If the bootloader is unlocked then they will not fix it as you do not have warranty anymore.
If you have a locked bootloader there is still a chance.
Ask for it back. Hopefully you don't need to pay postage.
To unroot it you need to just put it through PC Companion and select "repair software". Google instructions if you need to.
Wipe all the data as there will still be traces of root (factory reset the phone).
The other way to unroot is to flash a stock firmware with Flashtool, but PC companion is easier so try this first
The only complication is they might take down your IMEI number (unique number of the phone) and blacklist the phone as rooted, BUT this is highly unlikely so definitely try sending it again after unrooting and full data wipe. Likely they will get it at the new place and think it's never been rooted as they will not be able to tell. I unrooted mine this way and didn't have any trouble using my warranty.
Just make sure the Bootloader is Locked and you won't have any problems. If you don't know what the bootloader is and never purposefully unlocked it then it should be locked since you got the phone so don't worry at all and forget about this. Some people root this way to use custom kernels.
Good luck. I hope there is no problem at the next place you send it.