Create a GL Liability account called Credit Card - PO/AP and set it up in checking account defaults as a "Checking Account".  Enter the PO and receive it to ABC Vendor.  Then when entering the invoice in AP-C, enter as a COD rather than invoice and use the Credit Card PO/AP account to pay for it.  This properly records the vendor purchase and payment and books a liability to pay the credit card company.
 
When you get your credit card statement, use the Check Register (GL-I) for the Credit Card PO/AP account to verify the charges on the statement.  Then when you enter the credit card bill in AP-B, the total of  those charges already processed through the PO system are  posted to debit the Liability account for Credit Card PO/AP and other charges such as travel and entertainment that were not processed through the PO system are posted to the expense accounts as usual.

1.  No real secret but I would choose a range of numbers different from your regular checks just so it is obvious on reports that they are from a different account.
 
2.  It is not necessary but using the Check Reconciliation is a good way to double check the charges on the credit card bill.