I think your comment above about magical thinking might be on the right track. Along with Vitaly Mijiritsky's answer about never hearing from the folks who read your advice and choose to replace the microwave, there are a couple of other psychological principles that might be in play here:
- Loss aversion / endowment effect: Once people own something, it's harder to convince them of the value of getting rid of it. "I don't want a new microwave, I want my microwave to work."
- The cost of making a decision on purchasing a new microwave might feel staggeringly high for some people, even if that's not really the case. Some people have a really hard time making consumer choices, and they're terrified of choosing the wrong option. Analysis paralysis.
What can you do to help that? Well, I think you can continue to offer clear language about your service fee, be upfront about how replacing might be a cheaper option than repairing... and then, if you care to, you might help some customers get a new microwave that's the best value in your expert opinion. Do you have a few microwaves that you could bring with you in your truck? Could your service fee include a discount on the new model, setting it up, and hauling away the old one? It might be worth testing that once or twice and seeing if it ends with a great experience for the customer.