If the transaction isn't showing in PayPal (which I'm assuming is the payment method you have tied to your gamertag currently), it doesn't sound like a Frontier issue. That sounds like a MS issue, as you are buying the points from Microsoft store.
Though as already mentioned, there are other threads on here where people are reporting successfully purchasing points (and I assume being charged for them), but the points not then being visible in the Frontier store.
I'm sure FD support will steer you in the right direction, they are a helpful bunch.