This is not unusual.
Even though this was a voluntary repossession, it is still subject to any and all of the fees, plus default/penalty interest, provided in the contract for a default.
The $967 was probably your deficiency amount at the moment of sale, that is, the difference between what was owed on the original loan and the amount they got at auction - minus the commission they paid the auctioneer, of course.
Since then, they've tacked on repo fees (yep, even if you turned it in), and probably attorney's fees and court costs for filing a deficiency - have you checked with your local courts to see if they sued for it? And knowing Santander, they hiked up the interest rate on all of this to an even higher point than it was, based on whatever is in your contract for default interest.
That isn't to say there aren't mistakes.
You could demand an accounting, a detailed list of the amounts due - but that is going to mean contacting them and giving them your current contact info and address. So I'd be sure there isn't a lawsuit looming first, since they'll need to serve you if so.
If they've sold it or otherwise assigned it to a collector, those guys would be interested in updated contact info too.
