Normally I do not do one time mowing in my lawn service business. Usually it is not worth it to wear out your blades and mower on knee high grass. I did one last week though as it was a simple lawn that was flat with nothing much to weed eat. I got the customer to prepay since it was an empty home and the lawn was overgrown (good recipe to get burned on payment). I may reconsider the one time stuff with the prepay rules in place and making sure it is not too bad (head high brush !) I can charge a high price not caring if they take it or not since it is not my normal business routine. Some people sound sketchy from the start so it is best to avoid work that your instincts say run from. This one worked out for me but I'll post some signs of sketchy client phrases below.
"I want to see how good your work is before I sign up for regular service".
"The last lawn guy just quit showing up" (usually means they just stopped paying)
"Cut it as low as you can"
"I will get you more customers, just give me a low price" (give a discount AFTER you see all that work)
"I need my grass cut today" (I don't sit by the phone all day waiting on a call to come work)