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When you converted to the single Endicia account, it enabled shipping via all USPS services currently supported under one Endicia account.
You should see the First Class services listed on the Ready to Ship page. If you had orders on the Ready to Ship page before you converted, however, it is possible that the system was trying to use the old First Class Domestic account for those orders. As such, if you click the little x next to the order on the Ready to Ship screen, it will send the order back to the Orders page where you can restart the shipping process. It should jumpstart the system to use the correct Endicia account.
You can see all of the details about the migration to a single Endicia account here: FAQs: Migrating to a single Endicia account
If you still run into problems getting a First Class Rate quote, please provide an example order number in the comments below and make sure the order is on the Ready to Ship screen with the Carrier, Service, and Packaging all selected.