You definitely do not need to pay more. The P-37D is an excellent melodica, one of the best out there. Your issue with C4 and F4 is highly unusual on a new Yamaha Pianica. Chances are that this is a matter of gaps of the reeds for those keys. The gap, the distance between the reed plate and the reed, is almost certainly too narrow or too wide. If you take it apart and examine those two reeds carefully, they will almost certainly be noticeably different from the others, probably too wide. This is not hard to adjust, and you can see descriptions of how to change the gap by searching “gap” or “gapping” in the Forums. On the other hand, if it’s not too complicated to return the Pianica, that would be the safest thing to do. But eventually, you will probably want to learn to tune and to gap the reeds.