I can assure you that I have personally returned more than a dozen "higher ticket" items ($300-$2000). Many were 3-5 years old. Having the receipt ensures you get the full amount, and having the accessories and manuals assures that they will not hassle you. The original box is a nice touch, but not necessary.
Sometimes they will try to give you a Costco cash card, but I always insist it goes back on my American Express. I have yet to fail in the last 10 years.
If you read their warranty it says "We guarantee your satisfaction on every product we sell by offering a full refund, subject to certain product specific limitations." The only limitation is computers (6 months) and I think cigarettes.
In addition usually at the return line there will be a sign that says something like:
It is helpful if:
The product is less than 30 days old You have the original receipt
Notice it is helpful, not necessary. Maybe SoCal is just better about it, but I have returned to 6 different Costcos over the years here, and never an issue.
Remember too, you can request a UPS label via email (takes fricking forever, like 10 days) but that way you do not even have to deal with a person. I did this for an iPod Nano, and it had been about 9 months. No problem again.