The woman using the Pennies, as well as most of America, needs to understand Federal law. There is no law that states pennies must be accepted.
Section 31 U.S.C. 5103
This statute means that all United States money as identified above are a valid and legal offer of payment for debts when tendered to a creditor. There is, however, no Federal statute mandating that a private business, a person or an organization must accept currency or coins as for payment for goods and/or services. Private businesses are free to develop their own policies on whether or not to accept cash unless there is a State law which says otherwise. For example, a bus line may prohibit payment of fares in pennies or dollar bills. In addition, movie theaters, convenience stores and gas stations may refuse to accept large denomination currency (usually notes above $20) as a matter of policy.
Merchants have no obligation to accept money for a purchase.
The simple reason is that US currency is only legal tender for debts. It is illegal to refuse legal tender for a debt. When you make a purchase at a store, there is no debt.