If the young lady lives in her parents home, don’t send flowers before Shabbos unless you are planning to visit the home at some point that Shabbos.
If you are not invited for a meal, but to stop by for tea and visit, or pick the young lady up for a Shabbos walk then sending the flowers to the house is appropriate, just make sure the card wishes the entire family a Gut Shabbos, not a card just addressed to the young lady.
If she lives on her own or with female roommates, don’t send flowers. It will be taken as a sign you are committed and an engagement is imminent.