The term 'flat charge' refers to a fee that is charged for a service or product without any additional costs. It is usually a fixed amount that does not change regardless of the amount of use or time spent on the service or product. The two crossword solutions, RENT and RATE, are related to this concept. RENT is a payment made periodically for the use of a property, such as an apartment or house, and RATE is the amount of money charged for a service or product. Both of these terms are related to flat charges, as they refer to a fixed amount of money that is paid regardless of the amount of use or time spent on the service or product.