Shipping Fee

Have ordered many items happy with service and products I have received up to now. The only thing that I feel could be better is the shipping fee, why does the shipping fee increase as the amount of the payment increase. I also feel that after an order is made and it is in progress, that I should be able to add items to that specific order. It has happened in the past that the day after I ordered, a new special is advertised but then when I order I have to pay another shipping fee but the items/orders are delivered by the same courier at the same time.