If you create an upcharge and assign it to a weight product (e.g. Laundry per lb) then you can now add that upcharge to the laundry bags in an order based on a total or a %.

If you need to create an upcharge for this, just go to Settings > Products > Upcharges and make sure to assign the upcharge to the relevant weight product(s)

Have a look at the image below for what this looks like when you want to assign an upcharge to the laundry bags in order.