Are you in receipt of commission as your source of income or are you paying commission out?
If receiving commission, then I would have thought you would get a statement of some sort every time you are paid. You can use the statement to agree the amount paid based on the sales made so you are sure you are getting paid correctly.
I would post the statement onto sage as a bank receipt, reference the statement up in order and file it away.
Or you can load the statement as a sales invoice to the relevant customer account and then post the receipt to match it off.
Not sure on the VAT implication though regarding commission.