add to cart button

[willemo]

Can someone please explain how to add a number of  product items to cart, but only add postage once ?

For example,

If i create 'add to cart' buttons with postage, then then every time the user hits one of the buttons, the postage is added each time.

Alternatively, if i remove postage, then no postage is added.

How do set up the buttons so that postage is automatically included once only?