I got a 64GB kit from OWC. Works great, but I think a paid a little bit more. I did not send in my original 16GB since I wanted to keep it in case I need to deal with Apple. I think they would have gotten me $100 back.
I have used Data Memory Systems since 1987. They are Apple Specialists, offer a Lifetime Warranty and are Great to do business with. In all those years I have yet to get a bad memory stick from them.