Confused about shipping choices in creating listings

[smart_choices]

When I create a listing it asks me for the item weight, and then when choosing a shipping service it has a choice to calculate charges. It then asks for package dimensions and service type. I'm puzzled why I would wan to NOT want to calculate shipping charge. Also, how does the measurement dimensions of the item affect each choice? I think that these issues are related somehow? Any simple explanations of how all this works? Thank you!!

[redmodelt]

If you want to use calculated shipping, you need to know what all those dimensions and weight is packaged for shipping. Even if you had free shipping (it is not free), you would still need to know above to cover shipping on the ad price for shipping to furthest point it would travel in the 50 states. 

How does knowing what measurement dimensions of item affect each choice. If item goes into dimensional weight USPS and other services (1728 ci for USPS), or goes over a certain length, depending on the value of the item, may not be worth selling because of cost of shipping. 

 You need to know that before selling when setting up the listings, not after. 

[caldreamer]

You have a choice of calculated shipping (based on weight & dimensions) and "flat rate" (some Sellers like to charge customer flat $10 for shipping).   Of course, if actual shipping costs "more than $10 you will be losing money.