Upfront Pricing VS. Time and Materials pricing

Whats the difference between upfront pricing and time & materials (hourly)?

Since I give my clients a free estimate I can give them an upfront price before I do the work, but my price is based on time and material. This “hybrid” pricing works great for the professional plumber (ME) who inspects what work is to be done, tells the client what to expect, and quotes a fair price based on individual jobs. I think this is the fairest way to charge a client.