I found a logic error in the calculation of shipping which makes it more expensive for the customer.

For example, that this scenario for mail (Canada for example):
Item A: 20lbs fits Medium Flat Rate box
Item B: .5 lbs, fits small envelope (6×9″)

shipping item A only: Flat Rate: Priority Mail International: $47.75
shipping item B only: First-Class Package International Service: $10.00
shipping 1 of each A + B: Flat Rate: Priority Mail International: $72.70

So, as you can see shipping is more expensive for 2 items combined compared to each item individually.

So, I think I figured out the reason for this error. By the way, its the same logic error for USA/Domestic also, I just used Canada as an example because one of my customers is just complaining about it to me from Canada.

The reason is: The calculation can correctly determine the cheapest rate for single items. For example flat rate box or first class mail for the examples above. However, when the are combined, it first calculates the Priority Mail rate for the item A (correct), however then it does not give the normal First-Class rate option for the second package. It automatically defaults to force Priority Mail for the second package as well, thus in most cases forcing into a higher combined price class.

So, somehow the shipping logic will force all packages in the order to be Priority Mail if at least 1 (for first?) package is also Priority Mail.