we have a home service business in area that does plumbing, electrical and hvac and they work on subscription base (pretty big company with lot of employees). basically to get us to sign up, they made the subscription only cost like an extra like $30 more than the one time service fee and then up charged to add the other services. we had them come out for electrical work and the subscription for electrical for the year covered the service fee if we ever need them again and was only slightly more expensive than it was for one visit. then they start in with "you can add hvac service for $xx" and "you can add plumbing for $XX"
id say figure out how to make it financially for you, and work backwards from there.