Entering payments made via a third party for supplier invoices

I have started using a broker (OFX) to pay our invoices from our supplier in Germany. To process this in MYOB, I was planning to:

• Set OFX as a contra ‘bank’ account
• Enter the supplier invoice as per normal
• Pay the supplier invoice from the OFX Bank Account
• Enter a purchase for the OFX transaction, including the supplier invoice total and their administration costs.
• Pay for this OFX transaction from our main bank account

My question is, I’m not quite sure how to enter the OFX transaction.  I assume I will need to make a purchase, but given the supplier invoice has already been raised and the purchase allocated to the inventory cost of sales account.  What account do I allocate OFX purchase to…