Incentive to encourage tenants to pay online

Hi,

First-time landlord here :) I just got a tenant into my condo (yeah!). To encourage them to pay online on a scheduled recurring basis, I was thinking of offering them a one-time gift certificate. What is an appropriate amount? Is $100 too little or too much? The rent is around $2,300. What are other alternatives that you would suggest? Thank you so much!