When you decide what to charge your customers for shipping, the best approach is to use shipping rates based on what shipping costs your business, in fact. The most accurate estimation of shipping costs you can get by using carrier-calculated shipping rates, or, as an alternative, you may decide you want to configure a custom rate.
In X-Cart, you can configure rules to provide flat rates for:
subtotal ranges (the shipping cost will depend on the order subtotal amount);
weight ranges (the shipping cost will depend on the weight of items in the order);
item ranges (the shipping cost will depend on the number of items in the order);
any combination of subtotal, weight, and item ranges (the shipping cost will depend on more than one parameter).
Note: You can also use flat rates to provide free shipping.
By default, your online store has several flat shipping methods pre-configured. A store admin can check the list of enabled shipping rates on the Shipping page in the Admin area.
X-Cart 5.4.x - Store Setup > Shipping
X-Cart 5.5.x - Store > Shipping
A store admin can remove shipping methods from the list and then add them back when required. It is also possible to configure new flat shipping methods in addition to existing ones.