I'm sorry that Costco told you incorrectly. They were only partially right. There is a code that you get, but it's not for Online TurboTax. That's not how it works. You have to phone a number and provide some info from your CD to get a download code to install the desktop software.
Review the information in this FAQ:
https://ttlc.intuit.com/questions/2590568-i-bought-the-cd-but-my-computer-doesn-t-have-a-cd-drive
If you have already finished an online return, you could phone TurboTax Support during business hours and see if they will provide you a service code instead so that you can stay in Online TurboTax, but I don't know what they will say.
Or you could ask them to place the download in your account, which might be quicker than you going through the process in the FAQ.
Once you have the desktop software you could download your *.tax2018 data file from Online and open it in the software.
If you want to speak to them, here's how. You can use the form at the link below to schedule a call back, or it may provide a number. Hours are 5AM-9PM Pacific (8AM-12 Midnight Eastern).
Where the form asks what product you are using, be sure in the dropdown menu to indicate "CD/download" and the proper operating system, then indicate which Edition.
https://support.turbotax.intuit.com/contact
