Pretty certain there is a difference between stretch goals and add-ons with any Kickstarter. Add ons are the options you can add yourself. Stretch goals are what the crowdfunders unlock, as the pledge totals increase.
I think (hope) the stretch goals will arrive with the physical Kickstarters, scheduled for this January. There should be an email for each PDfs, except sometimes you just need check your order on DriveThruRPG, and the order number would be updated with new PDFs, as they arrive.
