How to calculate customs rates for dropshipping with Amazon and Oberlo?

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Hello, I'm setting up my dropshipping store but I don't know what would be the situation if I dropship with amazon and oberlo when it comes to customs, I've contacted oberlo and asked them on that matter, they said that the customer is responsible to pay for the customs, when I usually buy from both Aliexpress or amazon personally, I don't pay usually for customs, I would pay usually for the shipment and that's it, and I know that usually the customs would be included with the shipping rate, but how would I calculate the customs?