Could this be done in eCart Calculations tab?
Wouldn't it be easier (possible?) to create a calculation in the eCart Object Calculations Tab to create a TotalCartWeight = SUM[TotalWeight] ?
Or do the calculations only work as line item for each entry in the cart, rather than as a Total Calculation on columns (like subtotal)?
Seems to me that this should be a logical calculation that would be built it since each item has a weight (and total weight for quantity ordered) and the Total shipping weight of the cart is what is needed to get an accurate shipping estimate from UPS or USPS.