For a renting holidays, the real estate agent will charge you a deposit to pay before your date of reservation.
Normaly, the rent is calculated on a weekly basis.
In case for your holiday rental you choose a bed and breakfast or gîte in France. Please contact directly the owner before to sign a contract and pay any deposit.
Before payment, be sure you have reserved the house or apartment you have choosen for your holiday rental.
Contact the agency to ask him for confirmation by email or letter concerning the availability, the total price, number of beds etc. the more details you ask, less surprises you will get. Do the same with the bed and breakfast's owner and gîte's owner.