There's a limitation of individually created download keys where you can't set the unlock price. As a workaround, if you create a bulk key group with 1 key in it, you can grant access to the extra files. Sorry for the inconvenience.
Ok, so "bulk keys" are the way to go instead of "download keys". But if you make just 1 key it can only be distributed to one person, and you have to make a unique key for each person? Having just one link would be helpful. And it makes you have to have an itch account and claim the key to download the files...