So just so I understand. You bought the collectors edition which includes this and the next expansion, but you only got a $20 credit? Do you still get the next expansion or did you also get $20 credit for that as well? I assume not and I also assume you won't be able to get that next expansion. So, even if you got a $20 credit, you still paid for the next expansion and likely won't be able to play it either. Sounds like a rip off. But I am glad you got something back. Even if it's just a credit on PSN.

Purchases and Redeem Codes are specific for the item itself, therefor the Expansion Pass would be a $35 refund since it shows up as "Expansion Pass" in your add-on list (PS4). Buying only The Dark Below would be a $20 refund.