I've done this twice, I hinted at the transfer in the initial e-mail correspondence with the first and she said "don't worry about it, we'll take care of it in the room"...the second gave a hint on her website and asked that the gift be passed to her in a giftbag.  I placed it in an envelope with a copy of my license slide it into a gift bag with a I-Tunes gift card, candle, and quality body lotion...she got her donation discretely and some goodies as well.

Bottom line, ask how she would like to receive the donation...just asking may suffice at making her feel more comfortable.