How to set boundaries with house cleaning clients

House cleaning clients can be great, but scope creep is where the job starts losing money. "Can you also do the oven real quick" sounds small until it happens every visit. I have learned to name what is included before I start and price add-ons before the bucket comes out. For cleaners who work solo or with a small crew, how do you set boundaries without making the customer feel nickel-and-dimed?