U.S. companies set minimum prices to halt discounting

solution exists such as, coupons, customer onboarding rebates, gift articles.

It would probably have to be other than the primary purchase, but if you know your customers, and the landscape of your sales deme, you could offer a range of most probably useful items to include "for free" along with the purchase.

if someone buys, monopoly include coupons for a case of soda. if someone buys, toothpaste, include free toothbrush, viceversa.

Surely this is illegal! Cooperating to set prices higher than the marginal cost of production is bad for the market, and hence, illegal.

This also violates the free market spirit. They're not competing, so consumers do not get the most benefit.