One question about this: How do most internet plans work with this?

If someone connects with a request, is that metered as ingress/downloading, and if your server responds, it is metered as an egress/uploading?

If so, that means an unlimited plan is a must, and even then, might ISPs flag you for abuse if large frequent requests/responses are being received/sent by the server?

In general, many (most?) internet plans specifically prohibit running a web server with a residential account. A business account would be necessary in these cases.

Are there really any non-flatrate plans left? And if I'm getting fined for using all bandwidth that I'm paying for, what am I paying for?