Variable products

Hi!

I have a question about a webshop from my client.
She is selling leather handles: http://www.bijlien.nl/shop/categorie/leer/
There are 4 different sizes and you can choose to add extra bolts with the handles. This goes up to ten bolts. With each extra bolt you have to pay €0,50 extra.
I made variable products and set every possible variation of the handle (size and how many extra bolts). This works fine, but it goes wrong when you order multiple handles. If you want two handles with 3 extra bolts you pay two times for the bolts in stead of once. I don’t know if I have to make an extra configuration or something. Can you please help me?
Thanks!

Kind Regards,
Marloes

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