Database Design for Credit Purchase
You can create the table - Transactions: po_id, client_id, transaction_type, transaction_purpose_id (you can create the table transaction purposes (products, services, taxex and so forth) amount_paid, transaction_status (processing = 1, proccessed = 2, failed = 3, unknown = -1 and so forth). 

Now, a clien can pay for an order however he wants. Taxex can be paid as well.