In Yaakov's brachos in Parshas Vayechi, he says to Yissachar
Yissachar is a broad-boned donkey, lying between the boundaries. He saw a resting place, that it was good, and the land, that it was pleasant, and he bent his shoulder to bear [burdens], and he became an indentured laborer.
The Ibn Ezra, echoed by the Abarbanel, says that the tribe of Yissachar did not join the army. They were deeply bound to their land, and psychologically unfit for battle. Because of the inequity of their not participating in the wars fought by Klal Yisrael, they paid a high tax to cover their share of the war effort- or they gave a tribute to placate threatening neighbors.
The Sforno says almost the same thing. But with his addition of one new idea, he presents Yissachar in an entirely different light- that their unfitness/unwillingness stemmed from their preoccupation with and passion for learning Torah.
from Sforno in 49:14-
Netziv in Vayechi:
The Netziv had previously presented this idea in his introduction to his pirush on the Sheiltos on Vayikra.
A few things I need to point out about the Netziv.
☛A. If the Yissachar/Zevulun arrangement were still in force at that time, then the fact that Yissachar paid for their share of the war and for their military stand-ins couldn't be called "a tax on Yissachar." It would cost nothing to Yissachar and would just increase the burden on Zevulun. It must be that Yissachar did work to support themselves, and they worked twice as hard in order to pay the tax, but this left them the menuchas hanefesh to learn.
This explains why Yissachar paid the extra tax but Levi did not. Levi was supported by Klal Yisrael, so it wouldn't make sense to tax them. The burden would just devolve on the rest of Klal Yisrael. Another difference is that Levi did not receive any private agricultural land in Israel, so their exemption would not be seen as inequitable and would not arouse resentment.
-Important note: Eli, in the comments, tells us that from the pesukim in Divrei Hayamim it appears that David Hamelech ended the exemption of Shevet Levi as well, despite the fact that the Leviim were always supported by others and its members never received any private agricultural property.
-Another important note from Eli in the comments: Eli directs our attention to the Medrash Rabba 99:8-9, here and here, that indicates that there are two models of Zevulun's support of Yissachar. One is the money-in-the-envelope model, which is what I assumed, but the other is that Yissachar grew produce and delivered it to Zevulun, who then marketed it far away where it was more valuable. Zevulun was the entrepreneur who spent his time finding and exploiting markets, traveling and handeling, while Yissachar worked close to home. If this was the case, then the tax might have co-existed with Zevulun's "support."
☛B. People ask on the Netziv from the Rambam (6 TT 10) that says that Talmidei Chachamim cannot be taxed. The words of the Rambam:
☛C. I posted this discussion for informational purposes only. Any attempt to derive political capital from these sources would be self-defeating. Even if the Netziv supported one side, all that would happen would be that the other side would passel the Netziv, just as they passel the Tiferes Yisrael and the Torah Temimah and the the Maharitz Chiyos and Reb Yosef Ber Soloveitchik and on and on. But in any case, the words of the Netziv simultaneously support and contradict all current positions. From the Netziv we see three things.
☞1. Preoccupation with learning Torah was a valid reason to avoid military service, even for large numbers of people.
☞2. People availing themselves of this exemption had to pay a very costly tax to fund their replacements.
a. Chaim B. points out that the Netziv in the Sheiltos makes a very important point, which he left out in his pirush on Chumash. He says that this arrangement was made with the agreement of the other Shevatim: וכל אחיהם הסכימו להם. It was not unilaterally imposed upon them by Yissachar by fiat.
i. I think that even though it involved the agreement of the other Shevatim, they would not have agreed if they did not approve. If they hadn't approved, then every rich shevet would avail itself of this option of paying others to fight in their place. This indicates that the other Shevatim recognized the great value of what Yissachar was doing, but only to the extent that it deferred them from active participation. They felt that Yissachar still had to pay for their replacements. But we cannot deny the fact that according to the Netziv, if the other Shevatim would not have recognized the value of learning, even if this was because their hashkafos were flawed, Yissachar would have no right to unilaterally impose their wishes on the Klal.
ii. Chaim's point might answer the question from the Rambam in B. The tax exemption for Talmidei Chachamim is not relevant to this "tax," which was a negotiated alternative to military service. Proof for his excellent answer is from the Gemara in BB 8 (הכל לכריא פתיא אפילו מרבנן) that even Talmidei Chachamim are obligated to pay for local improvements and digging wells, because they directly benefit from them. They only don't have to go out and dig themselves, but they would have to pay others who do their digging for them. This is reflected in the Rambam's words as brought above in B, תלמידי חכמים אינם יוצאין בעצמן לעשות עם כל הקהל בבנין וחפירה של מדינה.☞3. While this practice pertained throughout the time of the Shoftim and the time of Shaul, Dovid Hamelech ended the exemption as soon as he became king, and all the Bnei Torah had to join the army.
a. People who were obligated to leave their homes and yeshivos to join the army of Dovid Hamelech could be sure that Kashrus, nekiyus hadibbur, Tzniyus, and Shemiras Shabbos were observed at a standard as high as their own. That might not be the case today.
b. Even if C-3-a is true, that doesn't necessarily absolve people from finding or creating an alternative service that would satisfy their needs.
It is clear that the tension between a life of immersive Limud HaTorah and military service is an issue as old as Klal Yisrael- perhaps even older, considering Chazal's "criticism" of Avraham Avinu's use of Talmidei Chachamim in the war against the four kings (Nedarim 32a, and see Sotah 10b.) This will never be resolved. One might even speculate that the tension is a formative dialectic of the character of Klal Yisrael.
Back to Tanach:
The reason I didn't mention the Radak is that I couldn't decide whether he belonged with the Ibn Ezra or the Sforno.
Finally, Rav Shamshon Rafael Hirsch tangentially mentions Yissachar's military deferment, but the reason I reproduce it here is the beauty of his paean to the Jewish image of an honorable life.
בהתאסף שבטי ישראל אל דוד אחרי מפלת שאול, באו מכולם אלפים ומאות אלפים. יששכר שלח רק מאתיים, את ה"ראשים" - השאר נשארו בבית ועבדו -