A typical order form uses one table for the "order" records and a related "lineItems" table to list the items ordered for each order. The typical layout is then based on Order with a portal to LineItems.
Is that what you have?
If so, you can use Sum ( LineItems::cItemWeight ), defined in Order to compute the total weight of your orders.
cItemWeight would be a calculation field defined as: quantity * UnitWeight.
UnitWeight, like a UnitPrice, would be looked up from a related table of products.