There are companies, mostly larger established ones, that require prepay on lawn services. This seems to vary by area though. If landscape maintenance is very seasonal (maybe only 25 mows a year) then it probably won't work. In my area we visit our clients 42 times a year not counting pruning or other extras. Because the service is so regular some companies are able to go prepay.
Prepay is not unprecedented. We all prepay for insurance, cable, phone, Netflix, etc...