Did you not take it back to the range and try it out some more?
You need to contact Glock again. You should not have had to pay for shipping but since you already have they may not reimburse. One member here paid for shipping then called Glock back after being advised by members here and all he got was a couple extra mags when the weapon returned. When I sent one of mine back I was emailed an overnight shipping label.
I sincerely hope you went back to the range a ran a few hundred more rounds through it vs just sending it back because if not you will more than likely be in the same boat when it comes back.
I've seen so many new shooters and new Glock owners give up to soon only to have same issues when the weapons return. Seen it several times on here also.
One of the Gen3's that had over 5000 rounds through it w/o any issue what so ever and on several occasions tried to limp wrist on purpose w/o failure........the first time I put it in my daughters hands she had 7 FTE's in 3 mags. After showing her proper techniques she hasn't had it or any other weapons fail on her since.