If I understand your question correctly, you can't. If you're thinking about the extra weight for packaging you need to account for that by reducing your weight bands.
When you set a shipping method there is a suggestion to factor in 10-20% weight for packing. For example my first weight band goes up to 80gm which allows 20gm for the envelope and grip seal bags and means I don't go over the 100gm limit for Large Letter. Took a bit of trial and error to get there, so you may need to tweak it occasionally.
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