Teresa
There is no with another item with calculated shipping. It is entirely based on the size, weight, mail class and zip code or country it is going to.
The rules - first, you need to have the dimensions in your listing for your item only - NOT the shipping box or mailer.
The weight should be the whole thing packed up ready to ship.
If your items are very small and light, and you often sell multiples, then it's a good idea to package up 2 or 3 and divide to get a more accurate weight for 1. Sometimes you do have to make slight adjustments.
And you need to list your mailers and boxes in package preferences so that the calculator can use the boxes you normally use.
By putting the ITEM dimensions in the listing, the calculator can fit the items into the best mailer or box you have listed and then it takes the weight you have listed in your listing, depending on how many were ordered, and adds that up.
If you get it set up correctly (this can take some trail and error) then the shipping should be accurate no matter how many items are purchased.