ok, well, you have 2 parts going on.
1) marking which items to include in the shipping rate calculation.
For this, you need to add a column to the cart object. for the sake of example, lets name the column "shippable"
In the add to cart button behavior, if the item should be included in the shipping calculation, set the shippable column value to 1, if not, set the shippable column value to 0.
2) Applying the shipping based on the total of the cart where the shippable column has a value of 1.
for each shipping charge, you need to create a separate rule on the shipping tab of the ecart object.
for example to charge $8 shipping if the subtotal of items marked as shippable is between $0 and $19.99 the rule would be created as:
$this->ConditionalTotal("TotalPrice", "shippable", "1") > 0
$this->ConditionalTotal("TotalPrice", "shippable", "1") < 20
Flat Rate Change of 8
the other calculation would be similar just substituting the correct amounts.