As previous posters stated, Costco price is final. And I find renting through Costco, I don't get the usual pressure sales to get their 'add-ons', especially insurance, not that it's a huge issue, I always decline. You can find better deals elsewhere, however most of those are non-refundable.
Also, Costco prices are in USD (I'm assuming you're renting at MCO), so if you use a standard Canadian credit card, the conversion rate is bad.
Add-on insurance is optional, provided you have coverage on your credit card or personal home/vehicle insurance. I rent often and over the years I've encountered the occasional agent who tried high pressure tactics regarding insurance, even insisting it was mandatory. It's not. If you use your personal insurance, you usually have to enter the insurance company name, occasionally the policy number when you sign the documents. Good luck.