Automated Plug-in Updates

I develop several free plug-ins for WordPress, some hosted in the plug-in repository, some not. However, I’m experimenting with a new premium plug-in model and I’d like some help.

Here’s how it works:

1. The basic plug-in is available for free on the WordPress plug-in repository
2. Additional add-ons are available for a small fee ($0.99) from an online store

The plan is to have the basic plug-in be 100% functional for the majority of users. The premium add-ons would provide additional features and functionality that is nice to have but not vital to the functioning of the plug-in.

The second goal is to allow users to upgrade their add-ons for free as updates are made available. Unfortunately, since I won’t be hosting the add-ons in the WordPress plug-in repository, this is much more difficult.

I will be hosting a separate SVN repository for these add-ons, so that’s not the issue. What I need help on is the actual process WordPress goes through to check for an available update to a plug-in and what it does to download and install the update.

This isn’t a well-documented operation in the Codex, and I haven’t yet taken the time to dig through the source. I’m hoping someone might be familiar enough with this system to give me a few pointers.