That would simply be PR or friendly copy. The implementation of nudge in your example would be to create the "discount" itself to push more people to pay taxes

That would simply be PR or friendly copy. The implementation of nudge in your example would be to create the "discount" itself to push more people to pay taxes on time. Even then, in your example everyone already understands and agrees with the rules of "pay a fee for being late" because it's such a common practice and is rational. Nudges don't need to be rational - at least not directly. They need to push people to do what you want them to do to attain some opaque goal. Similar to dark patterns, nudging is not about creating reasonable guides for people to follow, it's about making them feel less contempt for being shoehorned into taking actions they'd otherwise prefer not to. Maybe people don't want to pay taxes, but the proverbial "we" decided long ago that taxes are a necessity to maintain the state and so we punish people with prison time for cheating on them. In nudging, "we" don't choose anything, the institution quietly forces our hand without any democratic process.