upsell PER DAY, not just flat rate or percentage

The new upsell feature is wonderful but unfortunately doesn't work how I need it to. We charge a pool heat fee per day, not a flat rate or a percentage of the booking. Can you please add another option?

Fee type: add pool heat. This is VERY common in all of Arizona.

Amount type: PER DAY.

This would be life changing. LIFE CHANGING.