If the discount amount is more than the post and handling cost that appeared on the listing, or if post was free, you cannot discount an invoice to less than the total of goods on it. The total post cost is the only area that can be discounted.
To apply a discount if this is the case you could send to two invoices, one with the majority of the items on it, that is payable, the second invoice with the remainder of the goods that you wish to provide as free, and mark in the notes that it is the FREE quantity for multiple purchase and not payable, and then mark that transaction as paid for in your myeBay sales (so buyer does not mistakenly pay).
The alternative is to do a partial refund after the buyer has paid for all of the goods.
There is no way to reduce paying FVF on the actual freebies if the purchase has already been made.
If contacted prior to sale you could indicate that if a buyer purchases XX amount, then you will include an extra amount of items in their parcel and that this will be reflected in the invoice note, rather than the quantity being paid for.
