The gemara says that there is an issur to make pictures of the moon and sun do Rabban Gamliel could not have done it himself, rather he had a goy do it for him. Tosafos assumes that he commissioned a goy to make it for him, and therefore asks that by telling a goy to make it, it should be a violation of amirah l'nachri. The Ritva says that perhaps he simply purchased it from a goy, rather than have a goy draw it for him. The reason Tosafos assumes that he actually commissioned a goy is because he needed very particular pictures to show the eidim and it would be hard to imagine that he found exactly what he needed sitting in a store.
Tosafos answers that although there is an issur of amira l'nochri to have a goy do an issur for a jew, even something that is not shabbos related, for the sake of the mitzvah it is mutar. The difficulty with Tosafos answer is that Tosafos in Gittin 8b rejects the approach of the Ba'al Ha'itur (cited in the Rama O.C. 276) who permits a shevus of amirah l'nochri for the sake of a mitzvah without any additional shevus. How is it that Tosafos assumes here that we would permit amirah l'nachri on an issur Torah for the sake of a mitzvah? The Ritva implies within the approach of Tosafos that it is only for an issur such as this, the issur to create a form of the sun and moon, that we would permit it. The Ritva most likely does not mean that the issur of making the form of the sun and moon is in some way inferior to other issurim. Rather, he means like the Rashash who says that we only require a shevus d'shevus b'makom mitzvah to permit a more severe issur such as chilul shabbos, but for a regular la'av we would permit one shevus of amira l'nachri b'makom mitzvah. Based on the Rashash it should come out that for Yom Tov which is just a la'av, there is no misah or kareis, we should permit even one shevus for the sake of a mitzvah (meaning that Tosafos would agree that one can rely on the ba'al ha'itur).