Adding postage charge in shopping cart

Does anyone know how I add the postage charge for each item or the total postage charge for the whole cart to the paypal button ?

I do not know the weights of the items yet so can not use paypals system using the weight of the items.

I do how ever havethe cost of the postage for each item from other sources, so all this data is in mysql ready to be added up for the total postage costs.

But I do not know how to add the postage to the button.

any suggestions ?

This probably isn't the right forum for your question. What programming language are you using? I'll move your thread to the right forum.