Very complex shipping cost structures can be easily implemented as one shipping method by describing each by a simple rule with conditions. Shipping costs can be depending on:

-) Total amount of the order (with or without taxes)
-) Total weight of the order
-) Number of articles or different products in the order
-) Volume or minimal and maximal extensions of the products
-) Postal code of the delivery address
-) Coupon code

Each rule is described as a simple line of text with an easy structure (semicolons separate the parts of the rule).
The shipping cost can be given either with or without taxes included. Some simple examples are

Name=Free Shipping; 100<=Amount; 0
Name=Domestic Small; Articles<5; Amount<100; Shipping=1.50
Name=Domestic Standard; Amount<100; Shipping=3.50

This set of rules describes three shipping costs: Orders of 100€ and more are free, otherwise orders with less than five articles have shipping costs of 1.5€, all others 3.50€.

This advanced version of the plugin also allows shipping costs like 5% of the order amount, or 10€ per kg, or 2€ per additional article.
Name=Domestic Standard; Amount<100; Shipping=1.5*Weight

More advanced rules are employed by cargo companies, where the shipping per kg gets cheaper the more you ship. A (very complex) example is the following rule, which applies to all orders of at least 2 articles below 100€ and specifies shipping costs as 5€ fixed plus 3% of the order amount plus 1€ per kg plus 0.5€ per additional article:
Name=Complex shipping function; articles>=2; amount<100; shipping=5+amount0.03+1weight+0.5*(articles-2)

By adding a check for the coupon code in the rule, one can even create coupons that set a particular shipping cost. E.g.
Name=Free Shipping with Coupon; Coupon=="FREE_SHIPPING"; Shipping=0