If you are not using real-time shipping rates from UPS, FedEx or USPS, you can setup pre-defined shipping charges using this formula (Control Panel -> Shipping Charges):
SHIPPING = Rate + TOTAL_WEIGHT*Weight_Rate + ITEMS*Item_Rate + SUM*Percent_Rate/100
This formula shows how pre-defined shipping charges are calculated.
You can specify shipping charges based on order weight by specifying non-empty value in Range column in Weight field.
if you set up Weight to 5 lbs and Flat to 10 and leave all other fields untouched you will get shipping cost $10 for orders which weigh from 0 to 5 lbs, i.e.
SHIPPING = 10 (Flat) + 1 (Weight of order, for example) * 0 (Per lbs) + 1 (Number of items in cart, for example) * 0 (Per item) + 10.99 (Order cost, for example) * 0 (%) /100 = 10.
If you set up Order Total to 100 and % to 10 you will get shipping cost equal to 10% from orde r total if order total is from 0 to $100.
SHIPPING = 0 (Flat) + 1 (Weight) * 0 (Per lbs) + 1 (Number of items in cart, for example) * 0 (Per item) + 10.99 (Order cost, for example) * 10 (%) /100 = 10.