Hi,
Well, I still don't have a complete picture of the situation to be able to compare precisely the result on your website and the result from the calculator. You said "The weight I was using was 1.5 lbs and dimension of 16x10x1.5 inches". But I don't know which product on your website is configured with these dimensions and weights for me to be able to use in my cart.
Also, just the zip code of the destination is not enough for the calculator to provide accurate rates. As you can see in my screenshots, I had given a full address, with the house number and street name, city, etc.
I'm feeling the discrepancy you're seeing between your website and the calculator is because the information provided by the plugin on the website and what you provide to the calculator are just not the same.
First, I would recommend that you check the folder media/com_hikashop/upload/safe/logs/ via FTP. I would recommend deleting the logs already there if any.
Then, turn on the "debug" setting of your UPS OAuth shipping method in the System>Shipping methods menu.
Then, access your frontend, add a product to the cart, log with a user with a shipping address and display the shipping methods available.
It should then fill the log file in the media/com_hikashop/upload/safe/logs/ folder with the header, body and response from UPS.
You can download that log file and open it with notepad if you want to redact it before sending it to UPS' support.