The weights are cumulative and each item has to have a shipping scale assigned to it.
I reviewed your account and based on reviewing a couple of items I noticed that some of your items do not have a shipping scale. See example below:
& the shipping setting:
At checkout the system simply adds all the shipping scale values and compares it with the defined Shipping Methods. If the Minimum & Maximum scale match then the cumulative total will be added to the total cost.
I hope the above helps. Let us know if you have any other questions.
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