I definitely don't recommend online flowers--they're all shills for FTD, and you end up getting the same "Thinking of You (tm) Arrangement" that every other non-flower-literate schmuck ends up ordering. I expect you'll find LESS variety online than at a local florist.
What you do is, you call a florist in HER town (How would a florist in your town deliver them to her? They're not going to drive 10 hours for you!), tell them what you want and how much you want to spend. I PROMISE you, in a town the size of Detroit, you'll find a florist who can hook your girl up with some Gerber Daisies. Those aren't that hard to find. I assume she's said that those are her favorite? If not, BTW, then it's sort of an odd choice...
Anyhow, they'll take your credit card over the phone and find out where and when to deliver them, and what to say on the card (if you want a hard part, THAT'S the hard part). And if you let them know you're a flower virgin, they'll be extra gentle with you.
This has the personal touch, which is, frankly, the WHOLE POINT with flowers. Sending her a packaged, corporate floral arrangement just defeats the whole purpose.
Which florist in her town? Well, any of them ought to do a reasonably decent job. It's not Mother's Day or Valentines Day, so they shouldn't be excessively booked. A week notice ought to be PLENTY. I've ordered flowers with less than 24 hours notice before. If you know any of her friends (Or her mother! Curry favor now!) you could call and ask who's THE florist in Detroit.