Send options not working

['Woger]

I had some problems the last months with some orders being split up in 2 different orders both with their own shipping costs. I tried to find the problem, because it's getting really annoing now and customers are complaining and my "abandoned shopping carts" are tripled. 

I found out that as soon as I buy a small item which can be send by envelope (€0,85) and a bigger item send by package (€5,75) he counts up the sending costs and makes 2 seperate oders. I also found that if you buy 2 items which are the same it goes ok, but 2 different items it counts up the shipping costs....

In the end I found out, some items don't have "advanced stock" disabled. After I enabled this, shipping costs were back to normal.

 

This for reference if someone else encounters this problem.