Only one ACH payment at a time can be processed from the same payment method, and ACH payments may take a typical bank 3-5 business days to go through. If you try to initiate a new ACH payment from your bank while a payment is already pending, you may see the error "Outstanding payment waiting for confirmation."
We do offer a credit line feature at no cost that will eliminate the 3-5 day wait time, but in order to qualify, you need to have a minimum of one successful ACH payment. Once the pending ACH payment completes and you see the funds in your EasyPost account, please write to support at support@easypost.com and request a credit line, and we can get this set up for you.