What you are receiving sounds more like a contingent discount than a "commission payment"; i.e. a retrospective discount on the price you paid for the goods contingent upon you reaching certain criteria.
The correct way to account for this is not through an invoice from you to the supplier. You have not made a supply to them therefore you cannot invoice for any supply.
The correct way is either for your supplier to issue a Credit Note or for you to issue a Debit Note.
If you were entitled to recover the VAT in full on the original purchase of the goods, you can agree with your supplier that the credit / debit note can be issued without VAT.
If your supplier will not agree to this, you should ask that VAT be added on top of the discount you receive because your supplier will be able to recover the VAT shown from HMRC.
Senior VAT Consultant