FabFilter

Hi,

Has anyone purchased single plugins and then went to on purchase bundles paying only the difference?

It doesn't work that way in their store. This is what their store policy indicates:

Upgrading to a bundle:
If you think of buying additional FabFilter plug-ins, you may wish you had purchased a complete bundle in the past. This is why your account automatically creates a personal offer for you that allows you to purchase any additional FabFilter plug-ins with great discounts. You can use this to create your own custom bundle!