Well, great question and one that should be answered, however, there are some factors that the buyer needs to be aware of, one of them is, did the seller specified any "handling" charges..? are any taxes paid on that parcel...?, what shipping option was used to ship the item..? and possibly other considerations depending upon the Country from and to where the parcel goes.
Is my believe that bigger sellers get big discounts from the shipping companies but you are charged the regular price a smaller seller would pay that discount is part of their costs handling orders there is if they don't already specify a charge for "handling" and that would be double-dipping and very wrong in my view.
There are costs to the seller shipping parcels out, a box can cost up to $5.00, a padded bag a couple of dollars, bubble wrap rolls about $50.00 a roll, packing tape about a couple of dollars a roll, invoice pouches, warning stickers and so on cost money so, I cannot blame big firms to use the discounted rates to cover those expenses not forgetting the people that is paid to deal with the parcels.
Anyway, I agree totally that the shipping costs paid by the seller should show in the parcel/tag/sticker/label, is my belief that parcels coming from Australia do show or did show (Australia Post has made big changes to the system) those costs but I'm not sure about other countries, this thread does show that the USA does not show and it should.
Is not often you see a seller offers to the customer their shipping discounted rates as I did for many years and until a few months ago, even though as a small seller I have fulfilled the requirements Australia Post had at the time to give one of those discounted accounts that would only apply to International destinations, my request for such account was not to cover my packaging costs but to allow folks overseas to pay more affordable prices for the small orders I normally get, I passed on in full those discounted rates while I had them but don't expect all sellers to do the same for the reasons I explained above.
I don't care if a seller I buy from doesn't pass on the discounted shipping rates he/she may have, I understand the reasons as a seller myself even if small, I know what is excessive and what is not and if I have any doubts with today' online sources, it doesn't take long to find out how much money you should be charge to have your parcels posted/shipped to you, you only need to know the total weight of the parcels with packaging and if the listed items don't show the weight of the item, you can easily ask the seller or do a Google search and then add a reasonable extra weigh for packaging and you will endup very close to the parcel weight that will/was be sent to you.
Most businesses have automated shipping charges depending on the parcel weight and destination country/state, those (prices) are normally supplied by the courier company they are using, for example, my web-store shipping prices since I lost the discounted rates account are provided directly by Australia Post, there is, my web-store is linked with Australia Post where shipping costs are automatically calculated depending upon, total weigh, parcel size, destination, shipping option selected, insured/uninsured, etc., whatever that value is will show on the invoice that you will pay from, I, on the other hand, process the parcel making sure I add the packaging weight that can be up to 1kg in certain orders and when I process the label the parcel total weight by the gram has to be defined on the electronic form I have to fill up online (Customs, etc) so, unless I have covered each item I sold with a few extra grams for packaging, not only have to cover all my packaging costs but I will also need pay from my own pocket the difference between what the customer was charged and the actual price the system calculated I need to pay after the items were actually packed and this can easily be from 250gr to 500gr and that may represent $5 to $20 I have to pay more than what the customer was charged for and that not only sucks but eats any possible profits.
I'm sorry if it took me a "few" words to show you the 2 sides of the coin when it comes to shipping charges from sellers/suppliers, is it possible that some suppliers are overcharging customers for shipping...? I have no doubt but the majority has to deal with the issues I mentioned to make sure you are charged correctly/fairly and that you get what you paid for within a reasonable time frame.
Cheers
George