How do I create a coupon the is effective for one time charges

To be sure that your coupon is going to discount one time charges there are a few steps. 

In the coupon settings page located in Configurations> Coupons> Settings you need to enable the setting for one time charges and save the changes.  

Then when creating a coupon be sure that you are checking the box for One time charges in the section for Which charges are eligible for the discount, as well as ensuring that the in the At which level should this discount be applied that the Account bubble is checked.