Shipping costs are calculated for an order differently depending on whether or not the customer has already selected the shipping method for the order. In the MyCashflow default theme, the customer can see order shipping costs while selecting the shipping method.
If the customer has added products to the shopping cart but hasn't selected a shipping method yet, shipping costs are calculated based on the order's weight and the store's default shipping method.
The default method uses the first shipping method from the list on thepage.
After the customer has selected a shipping method, the order shipping costs are calculated based on that method and the order's weight.
Learn more about how available shipping methods are selected for an order at checkout.