Shipping cost for 1 or 2 products is the same

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Hi,

In our shop we sell CD's and the shipping cost is per product.
But now we do have a new product, a T-shirt in different sizes.
The price is the same for each size but.........
When you order 1 shirt, the shipping price is lets say $4,-.
The shipping price for 2 t-shirts is the same because they fit in the same package.
How can I set this up?

Regards, Theo