Just don't say ahead of time that you are tendering cash. Actually, it should be a reason to bargain for a lower price because there will be no charge for processing a debit or credit card processing. Once the sale of goods or services has been agreed, it should be unlawful to refuse Federal Notes to discharge the debt incurred.
I was at a dollar store recently with my daughter and the cashier asked debit IR credit. My daughter said cash. The cashier looked confused for a second.
It can be offered as it is recognized as legal tender. It doesn’t have to be accepted.
https://www.federalreserve.gov/faqs/currency_12772.htm