I started my '17 return, but never finished. I'm starting my '18 return and can't see '17. I could to pay $50 to get a plus acct. But can I pay for '18 at a deluxe level?

Yes paying for Deluxe in the same account will unlock it.  Deluxe is higher than Plus.  But you don't need 2017 to efile.  Since you didn't file 2017 enter 0 for the AGI.

How to finish a prior year online return   https://ttlc.intuit.com/replies/4186197

Thank you!