Shipping Issue

Hello, I am having a hard time getting open cart to calculate the shipping coast the right way,

for example if someone is buying and item A and the shipping coast is $10, then another item B and the shipping coast is $15 then the total shipping suppose to be 10 + 15 but I am not getting 425 instead I am getting 416 for example, please help.

Thank you.

What shipping method are you using? How is it set up?