Extras can be charged by indies, agencies and parlours although it is rare in indies because they set their own rules.
Extras are any sexual acts which are not the standard acts offered by each and every girl in the establishment... a normal service is for arguments sake a massage, oral (covered) and sex. If you want extras such as reverse oral, CIM, oral without you pay extra... it makes a type of pick and mix menu.
The extras are acts which are higher health risk and also at the discretion of the lady. Do you think it is fair for a toohless goon with onion breath to be able to kiss a lady or if a person who dislikes facials to have to received one?
The basic prices in parlours/agencies allows the guy to have a great service and if he wants anything else he can agree a price with the lady. It is no different then you deciding to book an indie depending on the type of service she offers... for example if Maddie offers owo, anal, reverse oral, and sex for £130 and Suzie offers owo, reverse oral and sex for £100 per hour... then you will go towards Maddie as she offers anal and you are feeling like anal even though the difference is £30.
Tango with regards to reverse oral a girl will charge extra for it if she feels comfortable with you and because it is not the normal service it would be unfair for a girl not to charge for it. It is nothing to do with feeling horney, it is just the rules set down by establishments which have a group of girls working for them.
I hope that makes a little more sense. xxx