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 via payment gateway or payment terminal 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.