Hello Rebecca - Thanks for your posts and excellent sleuthing!
Your ShippingEasy account includes access to two separate Endicia accounts. Each of these accesses different services, so until you register an Endicia First Class Domestic account, you will not be able to use Parcel Select Ground, First Class, or Media Mail.
This article provides more details about the separate accounts:
This guide can help you register the new account - and it is free to use - you'll only be responsible for the postage that you purchase:
Please post any additional questions in the comments below. Thanks!