As long as the company has an unsubscribe option and/or respects your response of "not interested", it is up to the company to nudge you to use the product and

As long as the company has an unsubscribe option and/or respects your response of "not interested", it is up to the company to nudge you to use the product and make the explicit decision to pay or not pay for the product. I don't personally see it as unethical as you have already demonstrated interest and have not opted out yet.