For a tree that size, don't worry about the flowers. The best thing is to make sure it survives. If the flowers drop and plant lives, it is better than the plant dying as it would hopefully live to flower another day.
Personally, I would take it back and purchase the other one (assuming it was indeed healthy looking) and start over. Do not cut or break up the root ball. Plant it 4" or 5" high, not level with the ground (if the area you are planting it in holds any amount of water when it rains, plant it even higher...avocados like to be watered but DON'T LIKE WET FEET). You can then mound up the dirt around the exposed part of the rootball. Water daily for about 30 days to help the plant establish itself. One other thing, do not add any fertilizer inside the hole when planting...I would also not worry about adding any fish emulsion right away. Biggest concern with a newly planted tree is establishing the roots.
While nobody has mentioned this, you are in St. Augustine and while the WM can take "cold" temps for an avocado, yo are pushing the limits of survivability for an avocado in your location. Be very mindful of that with the low temps and the duration of the low temps as you may need to offer it protection.