A merchant who accepts card payments cannot set any minimum amount for it. This is a rule of card associations, it is included in Comgate a.s. Terms and Conditions.
Fees and extra charges for card payments are prohibited since January 2018 by § 254 of Act No. 370/217 Coll. on payment transactions. This applies to consumer cards issued by Visa and Mastercard in the EU. Use of other cards can be charged.