Leave it with the donation...she'll see it and appreciate it. But, leave it outside the envelope and when you excuse yourself to wash up......she'll be happier when you return.
If it's flowers, candles or something that isn't hide able, give it up front and enjoy the rewards.

If you have already seen her I'm going to assume that she now knows that you are not a cop.  You can hand her the gift. It will be just fine.

Just give it to her up front...she won't mind, especially if you've already met her.  In fact, she will probably be very pleased...women like to be given gifts!