Jurisdiction Australia, but could apply globally for all I know.
Looking at major Australian bank websites, their EFTPOS contracts mention nothing about transaction fees being different for small amounts than large amount - it's a flat percentage (about 1.5% merchant fee).
Why then do they feel they can add add 20-30 cents (varies) if the amount is under $10-$15 (varies)?
The flat fee being added is applied to all credit cards even though most cards used in Australia are either MasterCard or Visa which don’t have a flag fall fee.
The extra fee is not added for debit cards, which in Australia are fee free for the merchant.