How customer pay for outstanding amount?

QUESTION: 

"How can a customer select to pay the credit back? Please explain the steps that the user should do."

Or "Let say a customer goes over finishing their credit. How will they pay for the outstanding amount"

ANSWER:

--> As you may know the module makes good use of the default Magento 2 purchase order payment method functionality so that the process or the logic of payment will base on this method of Magento default.

- About the Purchase Order, it allows commercial customers to pay for authorized purchases by referencing the PO number. The purchase order is authorized and issued in advance by the company that is making the purchase. During checkout, the customer chooses Purchase Order as the method of payment. Upon receipt of your invoice, the company processes the payment in their accounts payable system, and pays for the purchase.

- PO number is a series of authorized numbers issued by the seller through Magento 2. This number allows a business to make authorized payments for their customer orders.

- It’s recommended to only use a purchase order with trusted or qualified wholesalers.

--> As a result, You can choose any convenient method for the customer to pay back the credit, such as Bank Transfer or Paypal, ect. You could add this information to the email reminder sent to the customer for them to make payment.

Please refer more about the Purchase order here: https://bsscommerce.com/knowledge-base/magento-2-b2b/magento-purchase-order/