I'm not sure i follow why the weight of the clothing ordered would not be known at checkout time.
surely a pair of pants has a set wight?
in power store, when you add an item to the catalog, you can set the weight for that item.
If you use Advanced options for the Size, you can even set a different weight for small and large.
the shipping is based on the total weight of items in the cart, you can set your own shipping rates based on the weight, or you can use one of the rate lookup services like USPS to look up a shipping rate quote.