Woocommerce Product Add-on plugin

Hello,

We’re currently using thw woocommerce product addon plugin but it’s not achieving exactly what we want.

We have two checkboxes per product, each add an additional $20.00 onto the product, however we want it to only add $20.00 per order of that product, regardless of the quantity a user puts in.

For example, the product may be $2.00. If they select 3 of the product it will be $6.00. If they then add on one of the two optional extras it should come to $26.00, not $66.00.

Are there any existing hacks to get this done? Or alternative plugins that already achieve this?

Thanks