I Wool Knit
Maybe not be the most complex app but does what I need for free.
However, it does have some significant drawbacks.
The basic setting is 'unlimited instant downloads'. If you have mixed products which include a download plus a physical product, or if you want to limit the number of downloads per purchase, you have to reload the app page and edit each item individually. There is no way to change the basic setting - very annoying when you need it to change for every single item.