There are a couple of reasons why you may be asked to purchase postage.
You do not have enough postage in the account that you are trying to purchase postage for.
Since you have two Endicia accounts, it is possible that you may have added the funds to the wrong account. Learn more here.
You have the exact amount of postage you need for your current purchase.
Endicia does not permit a zero dollar balance on an active account. On the READY FOR PAYMENT screen, if you have just enough postage to cover the label purchase but you're still receiving the notification, you'll need to add postage to continue. We typically recommend leaving even just a few dollars in your account so you don't run into this situation.
If you are concerned about this happening and halting your shipping process, we recommend looking at how you can set up automatic refills on your account.