Sometimes providers offer an additional service for additional money. I had pre-arranged for just such an add-on prior to a session tonight, but during the course of our time together, it turned out it was not going to be available. I thought it would've been bad to remove money from the envelope on my way out, and I'm wary about any direct discussions of money in these situations, so I left the envelope total as is and I didn't say anything. Now I'm feeling upset over paying $400 for a $300 session.

What should I have done? Note that this is a well-reviewed provider and I'm not looking to burn any bridges with her. I'm just really disappointed with how things turned out and I want to learn from this so it doesn't happen again.

Did you discuss with her beforehand about the additional add on?  If so and she said that service wasn't available you should have let her know you put enough in the envelope to cover the add on and since it was unavailable you would like to correct the envelope.  Do this in plain site and allow her to recount the money if it makes her feel more comfortable.

but you just have to be open about it as breanna says.

any chance you are going to see her again, and maybe she will credit you that extra service?

(after you book) "hey, I'm embarrassed to ask, but on our last date I put enough in the envelope to cover xyz, can we try again this time?" or something like that.

Providers can run their business anyway they like and charging extra for certain menu items is one of those ways. I like a few things that can be extra with certain providers, so that's why I stick with providers with one donation fee and pass on the a la carte providers.  

I would strongly consider 2 envelops. Either put down the envelope for the primary fee at the beginning and give her the add on fee envelope at the end or leave both in plain site at the beginning. The risk you take then is she still make take the envelopes at the beginning and you may still be in a position to ask for the second one back.

The only time this happens to me is in the case where greek is involved. If I cannot perform it due to size issues, I always make sure to give the client that money back. I doubt she just forgot about it, but maybe she did not realize it was there until AFTER you left. I would offer you 100 off or whatever the upcharge was, in the future.

If you ever want to see her again, now it's a tip.

I am sure you would say no, as would I. Neither side should take advantage of the other. LR is dead on accurate with her response. The respecful, honest and decent thing to do is to give the gent his money back for the overpayment.

on her dresser.  Of course, I preplan the donation & have it separate.  I'd not put the extra for an up charge (like Greek) out initially.  If we are going for a Cruise to the Isles, I'm mention that I'd add it.  
First meetings are a lot of guess work.
HV

more time, to just hand the provider the extra funds at the end?
I certainly understand the discretion with the initial envelope, but wouldn't have thought that after the session there would be such concern with how you provide the additional funds.  However, if this is considered bad form I'd like to know, as I want to keep the ladies I visit happy (only two sessions so far - but after both I handed the lady some extra funds, once for a tip, the other time because we went over time a bit).