Advice for usability!

Hello, 

I offer a pet sitting service and for the service, I request payment for travel charges depending on client's location. I also offer a package deal where the client can purchase a service at a discounted price but then the client has to add the travel fee after. This stage seems a bit clunky as it often ends in chasing up for the extra payment. What would you suggest to make this more streamline?

Thanks, Ellie