I am pretty sure about something Rav Shach writes in Laws of Divorce 7 halacha 1 so I feel free to write down what I understand so far on condition that I might have to revise this. The issue is why a carrier does not have to say, "It was written and signed before me" in Israel. To Tosfot, the Sages were lenient because of an aguna. [That is a woman that is still attached to her first husband and thus can't remarry ] To the Ran and Rambam the reason the carrier does not have to say this in Israel is because it is not like laws of money. The difference is a carrier of the wife outside of Israel. To the Rambam, that carrier would not be required to say, "It was written and signed before me"; but to Tosphot, that carrier would. The reason being the requirement is we are afraid the husband will come and say the document was forged. The Gemara page 5 side b is a proof to Tosphot that if the carrier did not say it, then the validity of the document needs to be established by witnesses, or else it is not valid, So outside of Israel we are not lenient because of aguna, and the document needs validation from the law that all documents need validation before any action can be taken by them, not just a worry about the husband might come and claim it is not valid, So the reason for validation is like all document of monetary issues like Tophot, and not like the Ran and Rambam.
I admit that this requires some more thought, however it is what I think Rav Shach is saying,
[If can figure out what Rav Shach is saying any better than this, you are doing better than me. ]
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רב שך writes in גירושין ז' א'. The issue is why a שליח does not have to say, "It was written and signed before me" in Israel. To תוספות, the חכמים were lenient because of an עגונה. [That is a woman that is still attached to her first husband and thus can't remarry ] To the ר''ן and רמב''ם the reason the שליח does not have to say this in Israel is because it is not like laws of money. The difference is a שליח קבלה outside of Israel. To the רמב''ם that שליח קבלה would not be required to say "It was written and signed before me"; but to תוספןת that שליח would. The reason being the requirement is we are afraid the husband will come and say the גט was forged. The גמרא גיטין ה' is a proof to תוספות that if the שליח did not say it, then the validity of the גט needs to be established by witnesses, or else it is not valid, So outside of Israel, we are not lenient because of עגונה, and the גט needs validation from the law that all documents שנפרעים שלא בפניו need validation before any action can be taken by them, not just a חשש THAT the husband might come and claim it is not valid, So the reason for קיום is like all document of monetary issues like תוספות, and not like the ר''ן and רמב''ם
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רב
שך כותב בגירושין ז' א'. העניין הוא מדוע שליח לא צריך לומר "נכתב ונחתם
לפני" בישראל. לתוספות, החכמים היו מקלים בגלל עגונה. [זאת אישה שעדיין קשורה
לבעלה הראשון ולכן אינה יכולה להתחתן בשנית] לר"ן ולרמב"ם הסיבה שהשליח
לא צריך לומר זאת בישראל היא כי זה לא כמו הלכות של כסף. ההבדל הוא שליח קבלה מחוץ
לישראל. לרמב''ם לא יידרש שליח קבלה לומר "נכתב ונחתם לפני"; אלא לתוספות
כן צריך. הסיבה היא הדרישה היא שאנו חוששים שהבעל יבוא ויגיד שהגט מזויף. הגמרא
גיטין ה' הוכחה לתוספות שאם לא אמר השליח אז צריך לקבוע את תוקפו של הגט ע"י
עדים, או אינו תקף. אז מחוץ לישראל אין אנו מקילים מחמת עגונה, והגט צריך אישור
מהחוק כמו כל המסמכים שנפרעים שלא בפניו שצריכים תוקף לפני שניתן יהיה לבצע כל
פעולה על ידם, לא רק חשש שהבעל עלול לבוא ולטעון שזה לא תקף. אז הסיבה לקיום היא
כמו כל מסמך של נושאים כספיים כמו תוספות, ולא כמו הר''ן והרמב''ם
