Serach went to buy pita for dinner from a local establishment. She asked the person behind the counter, who's Chassidish, if they took the Lander discount card (a card issued to Lander students, kollel members, alumni, and their spouses), to which the man replied "I remember the guy came in here... you know what, yes, I'll take it." He promptly took off 10%, leaving the bill at about $3. Serach then asked about paying using a debit card, to which he replied "Why not?" When she asked if there were a minimum, he noted simply "No, that's illegal."
This is in stark contrast to another establishment which was happy to take the card... but only if you paid cash. As the owner told Serach, "If I take credit then I have to pay taxes on it!" Serach replied that he has to pay taxes either way, and left.
The best way of putting it: A family friend of my in-laws (IIRC) tells customers who ask about tax, "I don't charge tax. I just collect it."