Seller deducted from my refund, should I dispute?

I ordered a part for my car, and I tried to cancel the order within two hours of purchasing (before it was shipped). I found out that the person who was going to do it, wouldn't be able to make the repair. I figured that they'd cancel it because it's a large part, and shipping probably isn't cheap.

Well, the request was denied, and I got the part a few days later. I opened the box once, and it all looked great. I never took the part out of the box.

So, I started the refund process, sent it back in the same box that I received it in, etc. Today, I woke up to an email saying that "The seller deducted from the item price because it wasn't returned in the original condition. To learn more, see our condition of returned items policy." Out of the roughly $170 (including tax), $30 was deducted from the refund.

I know for a fact that I didn't damage the item, so I'm kind of like "bruh". I'm guessing that they did this to try to offset shipping, or as a restocking fee?? Should I dispute this?

EDIT: It does appear that they have a 15% stocking fee at the bottom of the listing.