USPS plugin decreases shipping price by 50%

-- url of the page with the problem -- : flordelicious.com
-- HikaShop version -- : latest
-- Joomla version -- : latest
-- PHP version -- : latest
-- Browser(s) name and version -- : Google Chrome

When I use a 10% coupon at the checkout (THANKYOU10) - it cuts the shipping rate in half. What can be the problem? Also - the USPS plugin calculates the shipping rates a bit higher than normal (but I manage it by putting a negative value in the Price field inside the USPS shipping plugin).
I am attaching screenshots - 1 is before the coupon applied and the second one is after the coupon applied, so you can see the difference in shipping price.

Also - here's my USPS plugin settings. By the way - do you guys happen to have a tutorial for the USPS plugin? I still cannot figure out how it calculates the price and I just have to randomly set the fields to adjust the final number close to the truth..
I am actually trying to set up the shipping price about 25% of the price of the item and increment for each additional item by 50% of the shipping for that item

Never mind, I changed the shipping settings and now it works, though I still don't understand why coupon was cutting the shipping rates...

Hello,
Regarding your USPS settings, you should not set a negative price option like -1.

do you guys happen to have a tutorial for the USPS plugin

You can check the USPS shipping documentation page to know what the USPS options exactly are.

I still cannot figure out how it calculates the price and I just have to randomly set the fields to adjust the final number close to the truth..

The USPS shipping plugin will send details of your order like :
- The shipping address
- Your order weight and dimensions
- Some of your USPS settings

And then it will display the shipping rates that are returned from the USPS platform.

I am actually trying to set up the shipping price about 25% of the price of the item and increment for each additional item by 50% of the shipping for that item

In that case the best solution will be to use a manual shipping method