Due to the recent financial regulation, we may see more stores providing discounts for those who pay with cash instead of with a credit card. Also, stores may decline credit cards for purchases under $10. However, one thing they are not suppose to do is to add a fee for using a credit card. But as this Consumerist article
describes, some stores are doing that.
This Ohio.com article has more details on this topic. The new rules that allow stores to set a $10 minimum only affects credit cards and not debit cards. However, I'm not sure if that means debit cards that are only used with PINs or if it also includes signature-based debit card purchases.