The default flat shipping module provided by Zen Cart only supports one shipping price. This module supports 4 prices:
1. Domestic Express Delivery
2. Domestic Normal Delivery
3. Small Items
Please note: #3 Small Items is based on Category (read here about "Shipping by Category or Product Name"
We also help you to rename the Shipping Method title to be any name you want. If you don't need "Small Items", price will be deducted to $45.
You are also welcomed to post your requirement to make your own "Flat Shipping Module".