I'm an exploder, not an exploiter. It's my nature. I'm way too honest and blunt to be an exploiter. (Fuck, here I am being honest about the fact that I don't think dishonestly is always bad and that for many minor forms of it, even though I could never pull them off myself, I'm ethically OK with it.)
But ask your local Googler whether I'd recommend whistleblowing as a career move.
In movies, people like exploders and whistleblowers, but in real life, people think of exploders as a pain in the ass (killing the messenger) while exploiters get ahead. I used to think that exploiters were all unethical, but I've come to realize that some people can be exploiters without being harmful or unethical.
You, not the retailer, are legally obligated to report your purchases and pay the tax.
You've stated your opinion about morality all right. You think it's okay to be a scofflaw (definition: "a person who flouts the law, esp. by failing to comply with a law that is difficult to enforce effectively"), even for tax evasion. Yet you'd look down your nose at someone lying to make ends meet. Tsk tsk!