ペットを持ち込むには１２５ドルの追加料金が必要だが

この記事を読むとペットのための１２５ドルの分も返還するとかいてるので
手続きを踏んで追加料金も払っていたようだ
United spokeswoman Maggie Schmerin said the airline was investigating the incident, and the family had been refunded the cost of their tickets, including the $125 pet cabin fee.
https://www.reuters.com/article/us-usa-airline-dog/dog-dies-on-united-airlines-flight-after-being-forced-into-overhead-bin-idUSKCN1GP37Y