I'm interested in developing for Ubuntu (mostly phones) and I can't seem to find the guidelines on app publishing, will apps only work through the ubuntu software center, or can users download and install an app from a website like is possible with an android apk?

Also, are there any rules regarding in-app purchase methods, (I hope the minimum price here isn't $2.99 in-app as well or I'm not going to even bother developing for Ubuntu and will just stick with Android) Google for example, requires that in-app purchases go through their servers so that it isn't possible to use other funding methods at least for play store published apps. My main questions here are: Would it be possible to release an app for ubuntu touch that accepted bitcoin, paypal, or other methods for in-app purchases? If not, would it be possible to release apps through a personal website or 3rd party app market that could use alternative payment methods?