It could be that he doesn't understand the tax system. He might come from a background where taxes were never really explained to him, and now in the fanfic scenario, he's clueless about how to go about filing them. He might be afraid of making mistakes and getting into trouble, so he just keeps putting it off.
Taxes in Harry Potter fanfic might not be a common theme, but they could add an interesting layer of realism. If there are taxes on magic use, it could lead to some characters having to find ways around it or deal with the consequences of not paying. Maybe certain spells are taxed depending on their power or rarity, which could create conflicts between wizards who use more powerful magic and those who can't afford to pay the tax.
He could face fines. Just like in the real world, not filing taxes usually results in financial penalties. In the fanfic, these fines could put a strain on his finances, especially if he's not expecting them.
One way is to create a new Ministry department that deals with magical taxes. You could write about how this department audits wizarding businesses. For example, Ollivander's wand shop might be audited for taxes on the sales of wands. Another way is to introduce a tax on magical creatures. Hagrid might have to deal with the new tax regulations for his care of magical creatures at Hogwarts.
In the fanfic, if Harry doesn't pay taxes, he might face legal troubles. The authorities could come after him, perhaps freezing his assets. He could also lose his reputation in the wizarding community, as it's an unethical act.
The plot could develop with the tax collectors in the wizarding world starting an investigation. Harry would then have to find a way to either prove his innocence or deal with the consequences.
I recommend the urban life novel, The Cultivation Writing System. The protagonist of this novel, Chu Dong, obtained a magical system due to an accident. He can obtain immortal cultivation resources by writing novels, including the chapter on collecting the "IQ tax." It seems that it should meet your needs. I hope you like this fairy's recommendation. Muah ~😗
The 'Don't Mess with Taxes' comic probably aims to educate and warn people. It might showcase various scenarios where tax evasion leads to trouble or highlight the benefits of proper tax payment through interesting stories and characters. Also, it could use satire or comedy to make the topic more accessible and memorable.