In your Zen Cart Admin in the Configuration ... Shipping/Packaging ... are settings for:
Country of Origin
Postal Code

You need those set correctly for where you are shipping from ...

Then, if you edit a Product, you need to check its Product Weight ... This is where the 5lbs or 3.5lbs is coming from ...

NOTE: Weight of the shipping quote is a combination of ALL Products in the shopping cart plus any weight on Attributes if you are using them ...