There are a lot of help pages on the team threads and on etsy help pages.
Are you creating your shipping profiles?
You need them before you can add weight/dimensions to a listing.
You also have to be using the New listing manager.
Profiles: You only need mail classes on profiles that you'll use.
If your items are all lightweight, a first class and Priority togther on one profile is all you need.
If your items are all 1 lb or more you'll need either a Priority profile or a Priority and Parcel Select Profile.
You can also add Priority express. if you think anyone would want this (I don't add it to my profiles)
Do NOT add media mail to a shipping profile if you are not selling books/cd's/etc. Since a buyer can pick media mail if you have it on a profile even if the item bought is not a media mail item.
Once you're profiles are set then you can add them to your listing and put the weight and dimension.
If you only use certain size boxes you need to disable Etsy box sizes and ENTER in your custom box sizes on package preferences.
Handling/package fee on shipping profiles is PER ITEM.
handling/package fee on package preferences is PER ORDER.
Total WEIGHT: item + any packing + box.
Dimension: item + bubblewrap/packing..
Links to more info:
https://www.etsy.com/teams/7718/questions/discuss/16257398/page/3https://www.etsy.com/help/article/6131?ref=help_search_result