Katia
If you are using CS you do not have the option to add cost for each additional item nor the option to put in any shipping cost. This is only for the old way of doing shipping cost.
With CS, the shipping is figured on the weight of ALL items purchased.
You do not enter a shipping cost, ONLY the weight of the item + packing + box and the item dimension (including bubblewrap if needed) when you list an item.
Do not enter the box size and make sure you enter the total weight of the item boxed and ready to mail.
You can only use "with another item cost" or put shipping cost down for listings without CS, done the old way.
No need with CS to figure out what additional items cost to mail.
CS does it for you.
You might have to tweak your weights some.
Example I sell goat coats, a few sizes I sell weigh with a box 1 lb. 10 oz.
If 2 are bought it calculates at 3 lb. 4 oz.
But I can get 2 of them in a slightly bigger box for 2 lb. 12 oz.
So what I do is put the weight 1 lb. 6 or 7 oz so it will calculate at the 3 lb rate instead of a 4 lb. rate.
There's a lot of info on this team on setting up CS and how it works.
Plus a link on etsy's help pages @
https://www.etsy.com/help/article/6131?ref=help_search_resultYou have to set up shipping profiles BEFORE you can use CS.
Be sure NOT to add all mail classes to one profile or someone could select media mail and the item won't be media mail.
Here's another link with info that Kathy and I have posted in the forums:
https://www.etsy.com/teams/7718/questions/discuss/16257398/page/3