As far as I know, the only fare you can get for Schiphol to Antwerp (not Thalys) is the walk up fare and is not cheaper if you book in advance. However, depending where you buy the ticket there may be card fees or ticket office fees. If you get it from the Belgian Railways site (recommended), there are no fees whereas from NS International you pay a fee for using a credit card. The ticket is available from the ticket machine at Schiphol (and any ticket machine in the Netherlands) but you may pay a 0.50 fee for using Visa/Mastercard.
https://www.nsinternational.nl/en/where-to-buy/stations-tickets-service-offices
says there is a fee of 3.50 for using the ticket office to buy international tickets which could be bought from the machine.