I don't see any option on my Software Center! I do not know if it is relevant, but I have created an Ubuntu One account (although I don't know how to inform the OS about it).
System Administration > Update Manager then click the
Settings button. In the dialog box that opens, switch to the
Updates tab, check the box next to the 'backports' repository. When you click Close, Update Manager will prompt to reload your software repository list.
You only really want to install a newer version of Software Center, so if you find that one in the list of updates (read descriptions), then install it and see if it has support for paid applications. If not, then you can safely repeat the above procedure and uncheck the 'backports' repository without having to run the full upgrade. And yes, in this case you'd have to either to a clean install to 10.10 or wait for the next LTS version.