Improve the umbraco installer
As I understand it, currently if there is an upgrade for uCommerce you have to install the new Package without uninstalling the previous package. This means that you end up with multiple versions of the uCommerce packages installed. That suggests to me it's less of an upgrade, and more of a new copy copied over the top of the previous. So, if there were files that existed in 5.1 that were deleted in the 5.2 build, they would not be removed. That means in 2/3 years time, when we have 15 versions of uCommerce installed, the filesystem and source control is going to be a bit of a mess.
Can I suggest that the umbraco uCommerce installer verifies if there is a previous version installed, and then properly cleans up after itself.