recently, here in the Bay Area, we have had a few losers leave empty envelops, short us, leave dollar bills, etc...  

Don't hand the donation to her or talk about it.
Just place it down and excuse yourself to the restroom.

I hate bringing it up, it is nice when they lay it down, go take a shower or something... I can put it up so that I can focus on fun...

Excuse yourself to "wash up" (which is always a good idea - you want to be clean for her).
With a regular, the envelope is still there when I come out (maybe she counted it or just trust me).
I strongly advise against handing it to her directly. I did that once years ago (with a future ATF) the smile on her faced just vanished as I said "this is for you"-  
Sometimes it is a much a fantasy for the ladies as for us so donations should be kept very discreet

Pick up a standard Thank You greetng card at the pharmacy or grocery store.  Slow down long enough to write a simple note in there, donation in the card, card in unsealed envelope.  

I write a personalized thank you message inside the card.

Every agency and independent I have ever seen has instructions for how they/she wants it done on their/her web site. Every one so far has said place it in a plain unsealed envelope. Place it on the counter or table where it can be seen but don't draw attention to what you do. Some say leave it on the bathroom counter when you go freshen up even if it's simply to wash your hands.

The thing is you should not be fumbling bills at her location.  If not an envolupe, some way to keep the correct donation together.  I use a paperclip.  I have the donation in my key pocket separate from my wallet.  On first dates I follow her instructions...  if none, I'll place it on her dresser & as has been said, step out to wash up quickly.    With Favs...  and I really like this set up...  they place a chair next to the dresser to recieve my clothes.  Joi was the first to be set up this way.  I don't like to dress with my pockets full of stuff...  good way to lose something.  So I empty my pockets onto the near corner of her dresser.  The donation goes on the bottom with other thngs over it.  When I redress, the donation is left behind.  Savy ladies are cool with it & t keeps the illusion going.    A young MILF, neriously swooped in to retrieve the donation before I had a chance to step away... but that was the only time.  
Once you have left the donation...  do not watch it.  i have noticed...  in passing...  that sometimes the donation is picked up before departure & sometimes not...  and with the same Gal.   More times than not...  in the back & forth of bathroom & dressing...  I do not see if she's retrieved it.    

-- Modified on 4/10/2013 6:17:42 PM

establishes exchange, and identifies it from you to her with intent. It is no better than handing it to her directly, for chrissakes.

.............providers who represent 90% of my dates as I impersonally just set down the cash next time.  Because you know, "maybe they flipped".

if needed.

set it so and so...and then ask you to take a shower real quick... and thats when I would count it....

...until I'd seen the lady at least once. I used the plain white envelope as requested. It's not bad or stupid advice though to say that's what I do since I clearly said do what the lady/agency says to do on her/their site. I should however have made it clear that my method is after knowing the lady. Newbies need to keep it simple.

What an experienced guy does with a lady he has seen before is a completely different matter that general advice to the noob.

just wonderin'