If several items are purchased, the buyer pays an additional 25p only
but if the total weight of the goods will reaches the next postal rate or their total value will be higher than £20-we will make adjustments to fit into actual weight category.

To get a discount, please do not pay individually, but wait seller's invoice with adjustments made for your purchases.
Please do not pay the invoice generated by the system right after purchases, if you made few purchases at the time. 
Unfortunately some seller's fee are not returnable to seller and you paid the system generated invoice, your discount probably would be less.