Ad Ons Adjusted to The Length of Booking

We run a podcast business, and a core part of the business is editing packages. So we charge £50 for a basic edit if someone records for an hour, and if someone books for 3 hours we would charge £150. Currently there is no way to do this in Cowlender and we can only have it set to one amount no matter how long the booking time is. We really really need this! I am sure many other people would benefit too!