You're both massively underestimating the interchange fees and overestimating their need to dodge taxes. If you had any idea how much money they made you'd know that they can offset all of that via writeoffs without having to even do any "clever" accounting.
They would need to be doing an insane amount of business to get fees that low. In reality it's anywhere between 7-15%, especially for small tx. When you combine this with the penalty system for chargebacks (which quickly escalates to not being allowed as merchants) and people trying small tx with stolen cards, this is why coffee shops, corner stores, etc charge a flat +% for any cc purchase and especially leverage an additional fee for small ones if they even accept them at all.
They would need to be doing an insane amount of business to get fees that low. In reality it's anywhere between 7-15%, especially for small tx. When you combine this with the penalty system for chargebacks (which quickly escalates to not being allowed as merchants) and people trying small tx with stolen cards, this is why coffee shops, corner stores, etc charge a flat +% for any cc purchase and especially leverage an additional fee for small ones if they even accept them at all.