why EBay taking fees on postal charges which are not actually made due to combined postage deal

[jasonsdad]

I sell individual Lego models at low prices and the majority of buyers tend to buy multiple items as I advertise combined postal charges.  As an example my postal charge is £1.15 per item, but often I can sell 6 to 8 cheaper items and put all into same envelope and still charge total of £1.15.   However as each item was sold separately 8 items would attract a total postal charge of £9.20.before I adjust via the invoice.  This is the amount E Bay levy their fees on,  so 92p.  However the customer pays just £1.15 in total so I am overcharged on fees week in and week out and I do sell a lot of combined items.   If this a fair and/or correct practice ?

   Thanks              Brian

[pixiesen2012]

you are only charged 10% of the P&P the buyer actually pays,

if you send a revised invoice the charge is made once the buyer pays your invoice