You can leave it somewhere visible so the lady will see it when she walks in.

Don't sweat it Jo,

If you leave it in a plane white envelope on a dresser near her entrance I assure you it will be gone (like magic) after her departure.

Good luck and let us know how it goes!!..DB8

place it there BEFORE she arrives. She will very likely want to check the amount (discreetly) since you are a new client... don't make her hem and haw till you produce it. That's very uncomfortable for the lady and could result in her walking out.

She'll arrive with street clothes, probably a handbag or suck in her hands.  Direct her to put her things on the table...  where the envolupe will be.  Or she might prefer to use the bathroom, where it would be left.
In either case, her web site will probably provide directions.
skb

and I place 2 kisses on the envelope.

I either lean the envelope up against the ice bucket or place it on the table in front of the sofa where will begin our date.