Double the rate if you specify a check-out date.

We are building a system for lodging reservations.
The price is “3,500 yen” for two days and one night.
(Set in Japanese yen).

When I specify the “check-in” and “check-out” dates from the calendar, the price is 7,000 yen.
It appears that the rate is calculated for two days (two nights and three days).

For example, if you click “check-in: 27th” and “check-out: 28th” on the calendar, the price will be 7,000 yen instead of 3500 yen.
the price will be 7,000 yen instead of 3,500 yen.

Is this a programmatic specification?
Or is it a wrong setting on the plugin side?

(*If I only select check-in, the price is fine, but if I don’t explicitly specify the check-out date, the add-to-cart button does not appear)

Thank you in advance for your help.