Before you start to go off screaming "breach of contract" you probably should remember that YOU fell behind on the payments and had the car reposessed. By the way if they come and get it or you turned it in, it is still a reposession.
If you have some other insurance that covers reposession you better drop everything and start looking through all of your paperwork to find it. However, The only "insurance" I can think of that you may be thinking of is GAP insurance. That does protect them if your car gets stolen, totalled, or other loss. But it DOES NOT cover you missing the payments and having the car reposessed.
The item I do agree with you on is the amount. If your numbers are right, then over $6,500 in interest in a year does appear to be excessive. But without knowing your original terms, and if in fact the "numbers" you have are correct it is hard to say for sure. However, in looking at the letter it seems as if this may be something else.
It does not appear to be a "collection letter", it appears that Santander may have taken over your loan and for some reason still show it as open. Whether this is an "error" on their part or with Citi is something that you may have to look into. Because what would be more logical is that if your balance was $7,500 an extra $1000 in interest in a year is not that out of the ordinary especially if you didn't pay. The problem is that if you call Santander you may start up collection procedures on the amount you do owe. So you better have all of your paperwork in order before you start to investigate this. It is also very possible that you are correct and citi forgave that $2,000 and you don't owe anything. But without paperwork to show that it is going to be hard(if not impossible) to prove.