Yea, I just read the part where he said it had been almost 2 years. If it happened in January 2016, then it's pretty darn close to 2 years. If it happened in December 2016, then it's only a year. Without a month I just went by what he wrote. Everything I suggested would hold true for either scenario.
The question sounded to me like he was asking for suggestions on how to proceed recovering his money. Considering the time span involved, I ASSuME that he is thinking lawsuit and asking for the next steps he needs to take to pursue one. I don't think they are looking to apply pressure to the vendor directly through this post.
The first issue is not verifying that you had received the refund when you canceled the order. That refund would appear in a week or 2 at most. If you had not received a refund in that time, you should have been following up with the vender via email to verify that they received your cancelation request and that a refund would be forthcoming. This communication should have been via email so that there is a traceable record showing you canceled the order and that the vendor's agent verified the cancelation.
If, after that, you still received the intercooler a few months later, believe it or not it would be yours. Law states that if something is mailed to your residence, it is yours. (Of course this was what I learned back in 1988 so laws may have changed since then...lol) HOWEVER, the RIGHT thing to do would be to reject the delivery like you did. The issue of a refund would be moot as that would have already been handled with the earlier correspondence and repeated follow up via email until it the refund was processed and received.
Of course, that's not what happened and in no way helps you where you're at.
All you can do now is have all your bank records in order (and by all I would have every statement from your payment method from the day the charge was put on the account until today) and all the shipping information from UPS. Now, I don't know how long UPS keeps tracking information on file, so you may want to contact them and see if that tracking information can still be recovered. Just having the tracking number may not suffice.
This proves 2 things.
1) You paid for the item and never received a refund for it.
2) That the item was not delivered to you and was, instead, returned and received by the sender.
Having all email correspondence about this order would be a bonus in backing up your claim that you canceled the order and the vendor agreed to a refund.
At that point, you present these facts to the vendor. If the vendor STILL doesn't refund your money, then you need to consult an attorney but at this point in time I have a feeling you may have waited too long to seek any redemption from the court system. Come to think of it, consulting an attorney may need to happen now IF you have all the above evidence in hand at the moment. This way, if your the window of opportunity to file a lawsuit is about to run out, you can file it before it does WHILE you're trying to negotiate a settlement with the vendor.
Communication here is key. IF you need to file a lawsuit because of time restrictions, let the vendor know that if they can have the money in your account by that deadline that you won't file. If not, you have no other choices. II would hate to see it come to that as I'm sure this is just an oversight by the vendor and nothing more. In lawsuits, only attorneys come out making money.