Props to you all, Princess Rose, Cambria, and tsvicki - the number of roses, the black bra, and the positioning of the undergarments found in the car are all elements of the crime I had never considered seriously. This is the kind of fresh, original thinking that may move this case forward - and as to the undergarments, that's the type of clue that was likely ignored by the all male Atlanta PD detectives who originally worked this case.
IMO, I can't find any particular meaning in the roses beyond the explanations you all have offered. I suppose, I see the roses as a further attempt by Mary's stalker to intimidate and frighten her, to somehow stake his "claim" to her. So far as I know, the roses didn't even contain a card identifying the sender. If it did, Mary didn't share it with anyone at her work place. She also talked regularly to her mother and her sister on the phone and didn't reveal the identity of the sender to them so far as I know - I suspect Mary didn't know who sent them. But the roses, the phone calls, the feeling of being followed and being anxious, were all part of the escalation of the obsession and stalking that culminated in Mary's kidnapping.
I find the discussion of the undergarments far more telling. Mary had been married six weeks earlier - if the perpetrator was an obsessed co-worker as the psychic suggested - the color of the bra may have no meaning other than Mary got lots of new lingerie leading up to her marriage.
The positioning of the undergarments, I also see as significant. The cut in the hosiery, the careful folding of some of Mary's undergarments, I see those things as consistent with the abductor acting out his sick fantasy of taking control of this woman he had become obsessed with - acts of domination and humiliation.
I can't help but think of the advice Oprah has given repeatedly on her show - if you find yourself in a situation where an attacker is trying to force you into a car - take a stand and say, "No! Kill me here if you have to but I will not go with you to some deserted spot where I'll be even more vulnerable." If only Mary could've heard that advice - she'd have been more prepared to defend herself and the outcome might have been very different.