Beeper is a bunch of Matrix bridges run as a commercial service. You can join the wait list or set up your own Matrix+bridges if you want (though that's more fiddling around).
As Beeper is based on Matrix, you can use the standard Matrix API, use libraries like Maubot, and plenty of others in other programming languages. HTTP calls are one way to send/receive messages but there's more flexibility to be gained if you pick a wrapper library IMO.