You can tell just how important it is for Cassandra to receive the news concerning her naval brothers, in Ja's opinion, as the letter is quite short,and therefore, would be relatively expensive to send, or more correctly to receive..

Just think of the information she usually manages to cram into a letter of one page's length.

It would be the same price to send one page crammed full of information,crossed and with extra paragraphs added , as it was to send this relatively sparse letter.

Sill, she was obviously just bursting to tell Cassandra of their brothers' news.