Coupons weren't exactly designed to collect surcharge but our customers showed us that it can be used for surcharges or even taxes (which we support by the way in another, formal way now!)

Indeed, if you need to collect extra charges, you create in Property Management / Coupon Manager an item which will, for instance, add 1% of the booking value to the booking. See here:

Next, you just need to apply it to the booking here:

Check also: 

Collecting surcharge while charging credit card