See http://www.istechsupport.com/faq.php, Item #5 under accounting.  You not only need to get the costs posted to the work order, you also need to maintain the AP Payment History that you paid the actual vendor and not just the credit card company.
 
The same approach can be used for voucher entries as well for non-PO purchases that you want to maintain vendor payment history.  Use AP-B, manual check and use the credit card "Checking Account" to pay the vendor.