Adding amount with a check box

From “Additional” tab, I tried to add a check box field that enables additional charge if it’s checked. I did add a field by checking “add amount” and entered an amount. The field appeared and I can put a tick but it doesn’t add an amount. The total amount doesn’t change.
Is this a bug or did I do something wrong?

The product page>http://erdsoap.com/product/custom-soap/

https://ww.wp.xz.cn/plugins/woocommerce-checkout-manager/