Is giving charity upon marriage with the intention of seeking blessings and increasing good deeds, which has become a common practice in society, included in "Whoever introduces a good practice" or is it considered an innovation (bid'ah)?

Charity is a good deed, and seeking ease in affairs through it is a permissible form of intercession (tawassul) by righteous deeds, as stated in the verse from Surah Al Imran and in the Hadith of the three men trapped in the cave. However, connecting charity with marriage in this manner, along with the assumption that it is a confirmed Sunnah, is akin to likening something un-Sunnah to something Sunnah, and there is a fear that it might fall under the category of Bid'ah Iḍāfiyyah (added innovation). As for performing it without believing it to be a Sunnah or obliging people to do it, there is no harm in that.