I am currently using the cloud solution and I would need that the commerce module somehow contacts an external server whenever a purchase is made. Ideally just a plain HTTP GET Request. Is this somehow possible? I can code a plugin myself if needed, but how do i do that using the cloud solution? The documentation advises to enable developer mode only in an offline version.

Thanks in advance.

You can't enabled developer mode in the cloud solution. 

You would have to develop it on a localhost install or different PHP server where you have upload access.

So I would need to purchase the offline version, code the plugin and then upload it to the cloud solution?

I guess then it's easier to code a smtp client and poll every minute for incoming purchase emails. Not to happy with a minute delay, but spending several hundred bucks for a single hook sounds overkill.

Yes, you would need to purchase a self host license just to be able to do that yes or hire someone. I know it's very inconvenient :/

Oh, hiring is an idea. Thank you for your response, that was very helpful.