Two final validations (flowers more than 20) didn't work because of missing brackets, otherwise good code.

100% => 

                if ((flowers > 20) && (season == "Spring" || season == "Summer" || season == "Autumn" || season == "Winter"))

                if (((chrysanthemums + roses + tulips) > 20) && (season == "Spring" || season == "Summer" || season == "Autumn" || season == "Winter"))

:-)

Refactored version:

using System;
 
namespace _03.Flowers
{
    class Program
    {
        static void Main(string[] args)
        {
            int chrysanthemums = int.Parse(Console.ReadLine());
            int roses = int.Parse(Console.ReadLine());
            int tulips = int.Parse(Console.ReadLine());
            string season = Console.ReadLine();  
            string day = Console.ReadLine(); 
 
            int workPrice = 2;
            double chrysanthemumsPrice = 0.00;
            double rosesPrice = 0.00;
            double tulipssPrice = 0.00;
 
            switch (season)
            {
                case "Spring":
                case "Summer":
                    chrysanthemumsPrice = 2.00;
                    rosesPrice = 4.10;
                    tulipssPrice = 2.50;
                    break;
                case "Autumn":
                case "Winter":
                    chrysanthemumsPrice = 3.75;
                    rosesPrice = 4.50;
                    tulipssPrice = 4.15;
                    break;
            }
 
            double price = chrysanthemums * chrysanthemumsPrice + roses * rosesPrice + tulips * tulipssPrice;
            int flowers = chrysanthemums + roses + tulips;
                
            if(day == "Y")
            {
                price *= 1.15;
            }
    

                if (tulips > 7 && season == "Spring")
                {
                    price -= price * 0.05;
                }
                else if (roses >= 10 && season == "Winter")
                {
                    price -= price * 0.1;
                }

                if (flowers > 20)
                {
                    price -= price * 0.20;
                }

                double finalPrice = price + workPrice;
                Console.WriteLine($"{finalPrice:f2}");
        }
    }
}