How to add thank you / welcome card to checkout.

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Hi I'm Ben, currently working for Urban Nature Culture Amsterdam. 

We would love to add physical thank you and welcome cards to every order done on our Shopfiy store. To do this our warehouse wants these cards to have an SKU so it can be order picked. Is there a way without using an app to add this product to the cart so that our warehouse can order pick it? And if so is there also a way to make this ''Product" not visible for the customer? I hope someone can help me out! 

Thanks in advance! 

with love,