Buyers are buyers because they don't know how to do something. It's important to be very understanding of their lack of knowledge. Also, when you price a gig at $5 you are only going to attract the worst type of buyers. I personally would never buy a gig from someone that prices their services at $5. I haven't done many gigs yet on fiver but I think my average earnings per gig is around $500 and I still feel that's under valuing my skills.
Also, never cancel the order or ask the buyer to cancel the order, instead seek free help, i'm an expert at excel and would always be willing to help out a seller that's for free that's having issues with one of their gigs.