Ask her why she has this feeling, this compulsion, to being such an independently social person, in terms of 'what friends do for others', that she is also incapable of accepting others (material/inconsequential/coupon) graces?
I have no idea if she is as far "un-welcoming" as some might be pick up on her being, but I ask, does she do this with family and close friends, when they may offer her gifts, or do you notice it just when it may occur to you, and your gifts?
In my cultural surroundings, I was taught from an early age that if someone offers you something, a friend, new neighbors, generally "good" people who surround you, the polite and most courteous way of thanking them is to accept their graces. Otherwise, it's seen as disrespect that you are "too good" to accept such offerings, or taken to an extreme, too good to be loved or cherished by others. It took me a long while to get this, especially with how Americans sometimes act when it may seem as though one friend is doing "charity" for the other one.
Let her know that your occasional gestures of goodwill, companionship, and love, is a sign that while such-and-such item (flowers, chocolates, jewelry) may be seen as trite, that in the thought you put into them, and the feelings you imparted upon them when thinking of how it might make her feel when she receives them... well, as soon as she tells you, '
no, thanks', do
those feelings you had become more and more deflated.