adding "handling charge" to shipping total

Now that the site is working, I have one more request (hopefully the last for this site...).
Jason helped me fix the problem of sending wrong weights to USPS. But now I'd like to use some calculation to send more accurate weights for multiple quantities.

This is the situation: Customers can only buy 1, 2, or 3 quantities of an item per order. All three quantities are shipped in the same box, which weighs 6.4 oz. A single product in that box weighs 18 oz., so the total weight is rounded up to 25 oz. Two products are 36 + 6.4 = 42.4 or 43 oz.; and 3 items are 54 oz + 6.4 = 60.4 (61 oz).

Currently I just have the single item 25 oz as the weight and multiply that by quantity to get TotalWeight. The problem is for three items the box weight is added in 3 times making the total 75 oz. When rounded up to pounds for Priority mail this becomes 5 lbs instead of 4 lbs for the actual weight.

I was playing around with various calculations without figuring it out, but then I realized that each item weighs the same (18 oz) and it's just the box weight that needs to be added to the total. I know some shops always add a flat rate to the postage for "handling".

At first I thought in eCart object / shipping I could just add the extra ounces, but it appears to be set up to add a specific money amount. I want the WEIGHT of the box to be added to the TotalWeight of the order and then have USPS return a shipping charge.

I assume there's a way in eCart to just add 7 oz to what ever the TotalWeight is for shipping, I'm just missing it.