Offering a discount to guests already booked

Hi

I’m not in the US but was automatically sent to this site. Hopefully you can help. 
Due to my desire to earn under the GST margin here I want to offer a discount to my customers who have already booked. 
is this possible and how do I do it. I’ve tried ringing BDC but they just hang up after a couple of rings. I’ve tried and tried. 
thanks in advance.