As we established in a previous Daily Halacha, the Sages forbade charging a penalty for late payment of an interest-free loan. Even though charging a late-fee does not transgress the Torah prohibition of Ri'bitt (interest), the Sages nevertheless enacted this prohibition due to the resemblance between such an arrangement and Ri'bitt.
This Rabbinic enactment applies only to loans; it does not obtain in cases of purchases. Thus, for example, a seller may sell a product for $1,000 but charge $1500 if payment is not rendered by the day it is due. Since the additional amount is charged as the price for a product, rather than for a loan, this agreement does not resemble interest and it was therefore not included in the Sages' enactment. (See Milveh Hashem by Rabbi Moshe Halevi, page 104.)
In another case, Rabbi Moshe Halevi (Israel, 1961-2001) ruled that there can be a problem of Ribitt which may apply to payment for utilities, such as telephone, gas and electricity. Companies often charge an extra fee for late payment, and thus if these companies are owned by Jews as is the case in Israel, of course paying this fee is forbidden as it gives the appearance of Ri'bitt. Rabbi Moshe Halevi therefore rules that Jews living in Israel should ensure to pay their utility bills by the specified deadline, lest they be required to pay the late fee. If, however, one does not pay such a bill on time, Rabbi Moshe Halevi nevertheless allows paying the late fee due to a number of factors, including the view taken by some authorities allowing paying Ri'bitt to a company or corporation (as opposed to a private individual). Preferably, however, Israeli residents should make a point of paying these bills on time to avoid the possible violation of this law. (Ibid, page 108.)
Summary: A seller may charge a higher price if the buyer pays after a certain date. Secondly, Jews in Israel should preferably pay their utility bills (gas, electricity, phone, etc.) on time, as the late fees charged by the companies might involve a prohibition of Ri'bitt (interest).
