Only charge one shipping fee multiple items

I can’t for the life of me figure out how to charge 1 shipping fee for multiple items. We sell a variety of paper maps, and if they buy one, i want to charge 15, but if they buy more than one, they shouldn’t be charged shipping because i can roll them up in the same tube. any idea how? right now i’ve set it such that if they spend 100 or more they get free shipping, which isn’t the solution cause i still need to charge the 15 for shipping.

thanks for any help

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