Next find out if you are actually dealing with a florist. Sometimes the online "florist" you find might actually be a middle man who will then pass your order on to a real florist. Why deal with a middle man. You are more likely to get what you want if you deal with a florist directly.
If they have passed the first two tests, you should check to see if the prices they offer are fair. There can be a great difference in prices between different florists. Check a couple of different websites for comparison. It should also be clear as to what you are getting for your money. look for actual pictures of the flower arrangements that you order so there are no questions or doubts.
Lastly, make sure the website of the chosen florist is secure. Online transactions take place with credit cards and you do not want to put your card number out on the net blindly. If the website is secure, the address will be "https://www" or there will be a little closed lock on the corner of your web browser. make sure one of those conditions is true before you enter your personal information.
As I said earlier, the internet has truly made our lives easier but we should not get lazy. Taking the time to verify that you are dealing with a quality business will greatly improve your odds of a successful transaction and a pleasant experience.