same-day special delivery
Refers to a postal or courier service that guarantees delivery on the same day the item is sent.
Please send this document by same-day special delivery.
速達 is general express delivery, while 即日速達 specifically guarantees same-day delivery.
当日配達 is a more common modern term for same-day delivery, often used by courier services.
Compound of 即日 (same day) and 速達 (express delivery).