Can I apply a coupon to a one-time charge?

In this case you do have the option to edit your Coupon Settings to allow coupon discounts on one-time charges (https://docs.recurly.com/docs/coupons#section-eligible-charges).

 

If you go to Configurations->Coupons, from the Admin Console UI, then click Settings on the top-right side of the page, you will get to the Coupon Settings page. Here you can enable/disable the setting to allow coupon discounts on one-time charges.

Once enabled, you will see a one-time charges checkbox under the Eligible Charges section of the coupon configuration page.