No commission

The questioner is referring to the credit card company's clearing fee, which is about 2.5%.
So his question is probably:
Some businesses prefer cash to avoid paying a clearing fee and many use cash to evade taxes. Should we be concerned about this?

In my opinion, it is unimportant and there is no need to be concerned, unless there is a suspicion that is not related to the method of payment, for example with an ultra-Orthodox business.

PS: You can actually just ask for an invoice/receipt.

No, it can't be, 2.5 percent is about 50 shekels, I'm talking about a difference of 400 shekels here... I think it's a way to report a lower income in order to evade taxes.

I would like to ask in response to this: Is it possible to buy, say, tefillin from someone like that, if I tell him I want to buy in cash, assuming he is also the one who wrote the tefillin. And are these tefillin kosher? By the way, he did not indicate to me why there is a difference. Am I obligated to ask him? Thank you very much and have a good and blessed week.

I told you there is a simple solution, ask for a receipt.

Are you interested in continuing to ask in theory for an imaginary case where you don't have the option to ask for a receipt?

This does not invalidate the tefillin. I already answered about the tax.