Our system will automatically calculate the rates based on the number of days.  So for example, let's sat you have an item with a 1-day rate of $10, 3-day rate of $25, and a weekly rate of $75 

If you rent the item out for 2 days it will be $20:

but if you rent it out for 5 days it will do a calculate like this:

5 days = 1 x (3-day rate of $25) + 2 x (1-day rate of $10) = $45

Or, for a 10 day rental:
10 days = 1 x (7-day rate of $75) + 1 x (3-day rate of $25) = $100