HOW DOES EBAY KNOW WHAT TO CHARGE?
I don't think they do and I doubt that eBay's combined shipping rules will ever work in this type of complex situation.
To make this work you need a shopping cart which can collect items and keep a running total of the weights. For additional accuracy you need to factor in dimensional weight if it applies. Although setting this up (writing the code) is not so complex it is beyond the abilities of eBay's system.
I can only suggest two things.....
Average things out to make it simple, monitor and tweak as needed to ensure you are not taking a hit (if your actual shipping charges are similar to your examples it should be easy).
Or
Forget trying to automate combined shipping, provide examples in your listings of the potential savings and quote specific amounts when asked or when invoicing.
The more varied the widgets you are selling the more problems, some things are best done manually especially with eBay buyers.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
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