Apparently you can buy Red Dead Redemption 2 on Epic for $10, but it’s slightly convoluted.
You buy Red Dead Online for $5 ($15 - the unlimited $10 coupon) on Epic, launch the game and buy the story mode upgrade (via the in-game store) for another $5. Altogether $10, versus $20 ($30 - $10) for RDR2 on Epic. I can’t see the upgrade price on either Rockstar’s or Epic’s store, so I’m taking the internet’s word for it.
I don’t know if you pay for the story mode via Epic or Rockstar store payment, but no one said saving $10 on a three year old game would be easy.