There are a lot of marketing and economic reasons why the manufacturer does not want to offer a discount on their own product even though third-party retailers may offer discounts. It's too complicated for a forum post like this, it's probably part of college-level economics. Although, I will mention that Costco makes almost all its profit from member fees, so they can sell products at or even below their wholesale cost.
If you buy the desktop product from Costco, and have problems, you can still get a 60 day money back guarantee from the Turbotax web site if you have your receipt.
https://ttlc.intuit.com/turbotax-support/en-us/help-article/intuit-account-billing/turbotax-satisfac...
