but I'm confused by your "subtotal is multiplied by the shipping percentage" suggestion 
well, that was a suggestion made when i did not have specific details on how you wanted the shipping to work, i had no way oif knowing that you where using a flat rate shipping fee. the details you provided in your response would have been very helpful in your initial post. It would saved us both from confusion.
anyway: yes, the product category approach will be the best one for your needs.
in the add to cart button for plants, set the category to "plants", in the add to cart behavior for stationary, set the category to "stationary"
you will need to create a shipping rule for each price break / category combination. the shipping rules will use a custom trigger.
here are examples of how to create the 4 examples you have provided (Note: these examples assume that the category column is named "category" and the values you are storing in the category column are "plants" and "stationary". remeber, PHP is case sensitive.) :
Name: plants up to 10
Triggers:
Custom condition:
$this->ConditionalTotal("TotalPrice", "category", "plants") < 10
Calculation:
Flat Rate: 7
Name: plants 10 - 15
Triggers:
Custom condition:
$this->ConditionalTotal("TotalPrice", "category", "plants") >= 10
Custom condition:
$this->ConditionalTotal("TotalPrice", "category", "plants") < 15
Calculation:
Flat Rate: 13
Name: stationary to 16
Triggers:
Custom condition:
$this->ConditionalTotal("TotalPrice", "category", "stationary") < 16
Calculation:
Flat Rate: 5.50
Name: stationary 16 - 32
Triggers:
Custom condition:
$this->ConditionalTotal("TotalPrice", "category", "stationary") >= 16
Custom condition:
$this->ConditionalTotal("TotalPrice", "category", "stationary") < 32
Calculation:
Flat Rate: 7.50
