I usually left a gift card out, sometimes I handed it to her

on the way out the door.

After the session she shouldn't be concerned about taking cash, but it could still be a little awkward, so I just left it in view.

They usually gave a nice thank you, the one lady who didn't see it texted later to thank me.

tc

If she went ahead with the session she has already decided that you are not LE. You can hand her the tip or place it on or near the envelope if you like. At that point it doesn't really matter.

Have the donation in your pants pocket.

When she comes over for a parting hug and kiss, reach in just before and palm the money.  As you break off from the hug, clasp her hand and transfer it from your palm to hers.

Watch the smile.

I hand the provider the money as I am about to leave.

It is sometimes said that the best tip is to make another appointment.  You can do this with a follow-up e-mail after your first date.