I think once again there is nuances to everything. If prices are well listed for everything, i would say kudos to the escort and there should be no bargaining. When it comes to extras, again some escorts list them fairly well or give you proper pricing by text and its awesome. This is easier obviously with indies as you know right away how much it will cost you.
If you book with an agency, often the booker do not even know (or won't tell cause of the YMMV factor or law factor) what extras may be offered. So for this specifically i think it does have a "free pass" in some cases. It happened to me once that a girl mentioned "no cim no cof" , so i then said "not at all or an extra could make it possible" ? Oups she then changed her mind and said cim could be possible. She mentioned 60$, i said "oh ok, sorry i don't think i have it on me as usually its 40$ in this agency" then boom she immediately said "bah...50 would do it" i said let me check if i have it , and in the end i actually had 50 in my pocket left. So i gave her that fifthy, even tough i could had probably kept dealing it to 40.... If i would had only have the 40$, i would had offered 40 and it would had been give or take... In that case do you still consider it bad to "bargain" ?
In any cases it always depend on ones point of view. I see more agency girls in general so this is rarely a situation for me, but when i will see an indy i would not bargain, especially not if her prices are clear by phone text and ads. Altough i do not consider it "a very bad thing" if some peoples ask... I mean who knows, it can be a quiet week for her and she rather take a booking for 20$ less than not have the booking at all. Peoples negociate in all sorts of jobs sector so why not this one? As long as they don't insist...
And remember here i spoke only of merb ads. Im not even touching the BP/annonce side of thing where girls will give prices, then change services, say they allow msog but then they do not, say they kiss and then won't do it. Whole different world... so if a service is supose to be offered and is not offered anymore on the spot, then i would say the guy should damn well bargain if he want to have it back in the package lol