Hello Jesse and thank you for posting to our community!
While all USPS postage can now be purchased from your single Endicia account, if you had orders listed in your Ready to Ship page at the time of conversion, they are most likely still locked into your closed First Class Endicia account.
To correct this issue, you would need to cancel the shipment and send the order back to your Orders page, and then recreate the shipment.
To accomplish that, simply click the X under the Options column in the Ready for Payment page. That will send the shipment back to the Ready to Ship page, where you can then click another X next to the order listed on the left.
That should then return the order to your Orders page, where you can start the shipment process over utilizing your new single Endicia account.
I hope that helps.
Thank you and happy shipping!