You have to remember that the seller is under no obligation to give you a discount on shipping after you have agreed to pay the prices stated in the listings. Should they work with you, yes of course, it is good customer service, but some sellers are either too busy or too greedy to care if you are offended that they charged you the full stated shipping.
The only way you can guarantee that you will get combined shipping is to message the seller and confirm their combined shipping rules before you bid or win the listing. Plus most sellers are more likely to respond to a potential buyer if they think it will help sell an item, so the odds of them responding to you before you buy is way higher than after you buy. If they don't respond to a question like that before you purchase then it is a red flag that you should may be not buy from them.