Custom package sizes based on shopping cart items

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I have been running Community Store with an EasyPost addon for a bit now and found that the package sizing is mostly fine for single products. However, when an additional product is added to the cart, I'm getting very overcharged for shipping.

For example: One shirt is shipped at .25 lb in a 1x6x10 envelope. One container of the product goes in a 1lb box at 5x5x5. However, if both are ordered, the EasyShip plugin will combine them into a 1.25 lb package at 10x6x11 to "make room" for all the products.

I can very easily roll up a shirt and fit it into a package closer to 6x6x8 so I'm getting overcharged right away for the packing slip. Is there a way to write custom rules to ensure that I'm getting the right package sizes based on what is in the shopping cart? EasyPost doesn't seem to allow you to change the dimensions after the order is placed so I am assuming it has to come from Community Store itself.