The marginal cost of the service they provide is like 10% of what they charge, or even less.

If you believe this, start a competing service. There is no moral or practical rule that forces prices down to cost without external reason. If Fastmail is making big bucks, and customers are happy with the price, that's good for everyone.

Do you go to restaurants expecting to pay only the total cost of ingredients for your meal? Fastmail provides an excellent service and if anything I feel as though I underpay for what I'm getting. I'm happy to know they have a healthy margin to pay their developers from and improve the product.

I expect to pay a reasonable price. Being overcharged by 90% is not reasonable.

Restaurants charge 4-5x the cost of ingredients. So I take it you never eat out?

I have worked in one and no they don't. Most restaurants barely break even by the end of the month once you factor in all costs (like employees, rent...), which I did in the example of fastmail.

You said marginal cost which is not all costs. Marginal cost is equivalent to ingredients and does not include fixed costs which likely make up the majority fo their expenses.

Any good alternatives you could recommend?