MysterD: More info here on that -
https://www.epicgames.com/store/en-US/promotion/mega-sale There's a section about ADD-ONS in above-said link, which is the section to read.
$10 coupons work on items priced over $14.99 that are tagged with the "ADD-ON" tag on its DLC section on the Epic Store.
This can include Expansions, DLC's, Season Passes, etc etc.
Coupons do not work on IN-GAME PURCHASES like Fortnite has.
MarkoH01: Unfortunately though it does not state if you will get multiple $10 discounts when buying multiple games >$14.99 in one purchase. I did not try this myself and used the safe route: buy control, pay, buy the season pass, pay ... worked.
I think if you buy something $14.99 or above and it is one package, you only get $10 off the package.
Meaning, like you said, you'd have to buy all the pieces separate to be safe - i.e. buy control alone, pay with discount, buy pass, pay with discount, good to go.
It would be nice if Epic Store got a Shopping Cart where you can toss multiple games, add-ons, etc into.