How do I adjust the invoice if I agreed to give a buyer a discount for buying in bulk?

[fisherofmen-1]

I agreed with the purchaser to reduce the total cost of the purchase if he bought 20 of the items I was selling. Is there any way I can reduce the total cost on the invoice, other than to delete a couple of items to bring it down to what we both agreed on? Would mean i'd still be charged commision on the full price?

[cq_tech]

No chooks, you're quite right. Only the postage component can be discounted, and only then by up to the maximum amount of postage applicable. I hadn't realised that the OP's items were 'free post'  (or at least I assume they are), in which case all I can suggest is that the OP cancels the original invoice then creates another one with say 18 items instead of 20, but sends 20 items to the buyer. That way they will only be charged FVF on the total amount of the new invoice.