usps plugin not calculating cost correctly

it appears to calculate shipping cost correctly on one item. But when someone orders more than one item, be it the same or different items, the shipping cost is calculated at much higher cost. Sometimes 50% higher than actual usps cost.

We have the weight entered for each item. Ex. an item weights 4 lbs. You order two of them and that is 8 lbs. It is normally charging double the first shipping cost instead of just charging for 8 lbs. So if one item was $8 then you order two and it charged $16.