Best way to build in postal costs for free postage?

[model-enthusiasts-uk]

Best way to  build in postal costs for free postage?

Our customers buy several item in one purchase where items are "free postage".

We want to reduce the built in postal cost per item when a customer  buys several item with "free postage"

If a customer buys 3 items (say) they pay three hidden postal costs that are built in.

We need to offer free postage to qualify for 

[tobiasd4]

You can't.

Ebay allow you to change P&P  on invoice, but only if separate to selling price.

e.g. item £2.00 P&P £3.20, so you can reduce invoice a max of £3.20.

With freepost, buyer pays total on all items.

You can refund some money after payment, but ebay won't reduce their fees.