FREE POSTAGE ON SECOND ITEM ONLY

[hats_hats_hats_hats]

I'm a seller. I want to set up a rule where buyers get free postage on the second item they buy from me, but NOT further items. I can't find how to do this.

The items I sell weigh the same, so buyers can benefit by having two of them shipped in the same box. However, adding a third, fourth, fifth etc item would require extra boxes and therefore extra cost.

Shipping would be something like:
1 item    - $10
2 items - $10
3 items - $20
4 items - $20
5 items - $30

...and so on.

Can anyone help?

[paper-jack]

Maybe would be easier to offer combined postage. I do this on another ebay account with hand made items. When buyers pay postage on more that one item (due to having ignored the offer or can't work out how to put items in the cart and request an invoice), I simply refund them for the excess postage paid. This is much appreciated by buyers and many return.