‘Deliver on time’ is not a tip. It’s a necessary principal for good business practices and long-term success.
As for outright asking if a client is ‘satisfied’ at the end of the transaction… (how to explain…)
Firstly, you should be checking in and/or reporting throughout the process, not just or only at the end. Be proactive rather than reactive. Second, asking at the end might cause a client to recall portions that were not pleasant.
Think of it like shopping at a store. You have your list and you find out the store is out of your preferred brand of beans. You get the house brand, because it works just fine. When you get to the casher, they ask ‘did you find everything alright?’ and instead of thinking ‘yes’, you just remember that they were out of your beans.
Now the store frequently has employees on the floor, but none of them talked to you. But you also didn’t seek them out. If you had asked, or if one of them had approached you, you might have found out that they did have the beans, they just hadn’t restocked the shelf yet.
Back to the cashier, you mention the beans. The casher takes a moment and looks it up and finds it. Yay! Except: you were ready to leave. You were ready to pay and walk out the door. You were fine with the house-brand beans. Now you have to wait.
This isn’t about satisfaction. This is about communication and human psychology. Consumer behavior.
Fiverr itself reminds/prompts the Buyer to leave a review plenty of times already. You don’t need to.