No, it is not ubiquitous. I really know a lot of different verticals and use cases and I would say the standard approach of using variant extra prices covers for sure more than 95% of all real use cases. But it all depends on a well thought concept of products and services and the proper use of the artefacts provided  by an ERP solution such as Odoo.

To come back to your example (long hair), you should well know why you want to charge an additional fee. If it is just for fun because it is maybe common practice, the customer will anyway not see a relation between the extra charge and the product. But is it based on a physical relationship (long hair need more color to be colored, long hair need more time to dry etc.) then there is a clear logic which should be properly modeled based on product variants, not only to charge the proper price, but also to properly allocate material consumption and labor time.

Thanks for your answer Ermin, this is actually the solution I came up with in the end, so I'm glad to hear it's a valid approach.

I was maybe not clear enough but you still understood what I ment regarding product variants, esp. the hair use case, this is exactly what we need (no it's not just "for fun";).

I created separate "virtual products" for modelling the variants, but they still need to be adjustable by hand, because every hair is different, but that's easily doable on the POS by overriding the price. You then clearly see e.g. "coloring" and the final price, so that's perfect for my usecase.