For delayed flights, we apply the seasonality based on the original departure date, so we will not refund the mileage difference.
Literal
Delayed-flights [regarding-については], original departure-date [to-に] based seasonality [object-を] apply [purpose-ため], mileage-difference [possessive-の] refund [topic-は] do-not.
Airline policy language dense with loanwords: オリジナル (original), シーズナリティ (seasonality), マイル (mile). いたしません is the humble negative of する — corporate keigo for 'we will not.' ~については (regarding) scopes the policy to delayed flights. ~に基づく (based on) is a formal modifier.