OK it looks like nobody has anything to say about this thread so I'll start off by saying what is obvious to me.
To answer your questions:

1. You don't have to tell a retouching service what you plan to do with your photos after they are finished, they are your photos and what you do with them is your business.
2. The costs of using a retouching service will add up if you want to do microstock seriously. However the stock agencies themselves will not sell your photos at a premium just because they have been through a retouching service.
3. Ideally you should find a service that will retouch your photos the way that you want, and be in constant communication with them - this could be an expensive option - but I would never send my work out not knowing what I am going to get in return.

Having said that I would advise against using a retouching service for a number of reasons:
1. It costs money that you are unlikely to make back on a lot of your photos.
2. You have a perfect oppourtunity to motivate yourself to learn how to do your own retouching.
3. Learning how to do your own retouching will make you a better photographer, knowing what can and cannot be done within your own skillset both in the field and in Photoshop.
4. There is no magical "make the photos looks more better". If you shoot a bad shot its just plain bad. If you shoot a great shot then you may not have to retouch it at all.

I recommend Lightroom and/or Photoshop.
Get the software, learn how to use them via millions of tutorials and forums on the Internet, save yourself money (the software will be cheaper compared to using a retouching service if you want a healthy portfolio).

I don't mean to sound condescending, and I'm sorry if I gave you that impression but here is a wonderful oppourtunity to learn and develop.
Don't rob yourself of a valuable life skill thats looking you right in the face.

I myself offer Retouching services to some of the top MS photographers already so I will suggest you to hire one. It will help you to work as a team and grow your port faster. You can earn multiple than what you will pay to retouching team. To avoid any mishap you can ask them to sign NDA before starting the job.

You don't have to pay extra as they will charge the same for that. I have used Retouching Expert services and they charged me same for that i.e. only 0.40$ per images for newbielink:http://www.retouchingexpert.co.uk/stock-photo-services.html [nonactive]. Quality was good and I am happy with the services they offer. You can also contact them for the same.