How much cheaper is it to buy luggage at the time of booking with Wizz Air compa...

When booking a flight with Wizz Air, it claims

It is cheaper to add bags now.

How much cheaper is it to buy luggage at the time of booking with Wizz Air compared to buying after (post-booking, either before or at the airport)?

It's pretty difficult to answer with any degree of accuracy, and it seems to defy logic, but the Service fees list tells us that, for a 20 kg bag, in low season, the price range is:

• Online: €20.00 – €112.50.

• At the airport: €70.00 – €80.00.

While in your case you are probably in high season (the range is €25.00 – €122.00 when buying online, but no price is given for at the airport in that case), the price quoted seems to be much lower than the lowest price at the airport.

Why, when and how it can become (much) more expensive online than at the airport is a mystery to me.