I need to import 000's of customers with a unique account identifier in the display name for each.

Hello northerngirl,

It's a pleasure to see you in Community.  I tend to agree, that having to manually input unique customer identifiers, can be rather bothersome when importing an abundance of customers to QuickBooks Online.  However, the option to include the display name isn't currently available with the import functionality.  

I have taken the liberty of navigating through the steps on my end, and found that editing the spreadsheet, and entering the customer identifier at the end of their name, did allow me to import them successfully.  I know this can also be a little time consuming, but it's a workaround for now.  Follow this link to see which contact fields will be imported and any restrictions that apply.  Please don't hesitate to submit a suggestion to our Development Team, if you would like to see additional import options.

Don't hesitate to reach out if you have any additional questions, comments, or concerns.  We're here to help!

@northerngirl 

How about using the migration tool?

https://get.transactionpro.com/qbo