Coupon value is zero but price is set up

Hi there,
I installed and setup the Flexible PDF Coupons plug-in and created a custom template. I create three product-coupons (two simple products and one variable product) and all of them have price set.

When I buy a product-coupon I receive two e-mails. One is the normal e-mail the users receives when he/she bought something on the shop. The other one is the e-mail regarding the PDF coupon. The strange thing is that in this second e-mail and in the PDF the coupon value is always 0 instead of the price set up on the product.

I tried to understand why is this happening but without luck.
Do you have any suggestions?

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