Johnny:

Version of OE?

To make sure I understand you correctly:

Good sold = $100
Taxes = $12
Total = $112
Fee = $10
Grand Total = $122

Amount posted to AR as the invoice total = $122

How is the fee calcualted?

Off the top of my head (having guessed at what you are trying to accomplish) I'd say it's possible using taxes and subtotals. If not, then I haven't met a problem I couldn't solve with PrintBoss.