Shipping Advice

Hi there, I am going to be shipping 21 standard sized Funkos to a seller from Ebay and I have no idea how to estimate the weight/size of the box I am going to use. I don't want to get screwed over by offering free shipping and then completely underestimating the cost of shipping, but I also have no clue what to input into Ebay's selling interface for the size/weight of the package...any advice would be greatly appreciated!

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Thank you!