I did when my Skrapnels came in realized it was cheaper to buy 2 singles and one Tailgat/Skrapnel set since I wanted 3 Skrapnels. They credited my account then when I did check out it ran my card for the difference.
It depends on when it came into your POL. If it was placed into it in the last 14 days, you should be able to cancel it for store credit. I've heard of people being able to cancel orders beyond the 14 days but it usually requires an email.
As far as I know you get store credit when canceling even after 14 days. I change my mind on stuff all the time and use the credit on other stuff.
I also have a question. Has anyone else not had their Cosmos/Swerve put in their POL? Skrapnel and Tailgate just dropped into mine, but Cosmos and Swerve still say preorder sold out on the item page. My preorder still hasn't dropped in. Anyone have this?