I agree that paying for return shipping on a warranty replacement before anything is determined is kinda bunk...
Sorry about your luck though... The Rena XP series really is a great filter if you learn all the little tricks to make sure it stays in top order. Make sure you push down hard & firmly on the valve assembly when you push down the locking handle. You'll never have this issue again if you do this. Like I said before, I almost did the same exact thing and it's an easily preventable issue. Don't give up on that XP3 yet!