This is the 4466th most frequent German word.
kostenlosen
Free of charge.
In this sentence, 'kostenloses' modifies 'Buch' (book), indicating that the book is free of charge.
Ich habe ein kostenloses Buch von der Bibliothek erhalten.
I received a free book from the library.
'Kostenlosen' describes 'Eintrittskarten' (entry tickets), conveying that the tickets are free of charge.
Die kostenlosen Eintrittskarten sind für die Veranstaltung am Samstag.
The free entry tickets are for the event on Saturday.
'Kostenlosen' modifies 'Versand' (shipping), indicating that the shipping is provided without cost.