How do I apply my pre-payment for 2018 TurboTax which was charged to my credit card in October, when I get ready to file my return?

[VolvoGirl]

Sorry the Desktop program payment will not apply to the Online version. Online and Desktop are 2 different separate things.  If you bought the Desktop program but used the Online version you can move your return to your computer and continue and file from the Desktop program without paying for online.  Or you will have to pay for Online and try to get a refund for the  Desktop program.

Did you get the program installed on your computer?  Here's how to move your return to it.
First download the .tax2018 file from the Online version,  
https://ttlc.intuit.com/replies/3300420

Then see, How to Switch to Desktop,
https://ttlc.intuit.com/replies/3302260

After you get the program installed the first thing to do before you open your tax return is to update the program and install any state programs you had. Then open your file. So you first might need to start a fake return to be able to download the state program (go to FILE - NEW).