If my checking on jisho isn't wrong then the Kanji used for "farewell letter" here can also mean suicide note, soooooo
To go into this a bit more, on page 13 we see Sayaka use the term 遺書 (with furigana いしょ). Which, according to jisho, can mean "note left by a dead person; suicide note; testamentary letter; will; testament", and is also the wikipedia definition of a suicide note.
Compare that to 遺言 (ゆいごん) which is defined as "will; testament; last request; dying wish" and is also the wikipedia definition for a Will in legal terms.
I don't anything about the cultural side of things but just going off of possible interpretative meanings, i feel like if Sayaka just wanted to have Yunimaru read a regular Last Will kinda testament, she would've said Yuigon, not Isho.