It's a monthly subscription. The price depends on how many flights you want. Each one-way flight is $105. E.g. So if you want four one-way flights per month it would be $420/month.
Yes, but it sounds like you pre-pay, and I would guess the unused flights do not roll over.
That's right, it's a monthly subscription (you're billed monthly). Unused flights don't rollover. But if you know you're not going to use some flights in a given month, you can send them to family or friends for free.