in-house dynamic dns server

We have a few customers on dynamic IP addresses that we would like to be able to connect to remotely. If we lead the customer through the procedure to find their IP address and tell us then it all works fine but I'd like something a bit smoother. The customers in this case are charities and other very low budget organisations so even then extra $10/month for a static IP address isn't feasible. I have a bind9 server, can that make use of standard dyndns protocols somehow (even via a separate daemon)? Or anything option would be good too. I'm getting a lot of noise on google when I try and search. Thanks James