customer service, and choice words are a learned skill.
much more challenging than wrenching, no doubt
especially in light that the conversation is some times plastic, in that often you can get into a conversation that has little definitive value.

so there it is.
initially
as you learn the ways of Customer Service
I'd stick to simple economic values. the specific cost of things.

I've been wrenching off and on for a bunch of years.
here in the US, Monterey, Ca. we start out with basic tune-ups at $65.

what this comes down to, is something like this.
a bike that has been sitting under a carport, or out on a deck somewhere, all rusty, etc...
and the customer wants it tuned up, new tires, and tubes.
that is easily going to be:
$17 each tire
$6 each tube
$65 tune-up

$46 in parts + sales tax at 7.25% (i think)
tax $3.34 (actually $3.335... round up = $3.34)
and $65 in labor (no tax on labor)
total: $114.34

i always do an initial inspection of the whole bike, point out any short comings (things that need to be fixed).
i.e. ripped seat, torn grips, broken plastic pedals, etc...

I'm quick to pull out a piece of paper, and jot down the costs.
things like tires, always fluctuate. Maybe one day we have Serfras Drifter, and maybe we get a bunch of tune-ups, then all we have left are Schwalbe Marathons, or something like that...

so maybe the in stock $17 tires get sold, and all that is left are $40 tires, or whatever they are.

on the repair ticket, I always make notes as to what needs to be replaced.

for a $65 tune-up
that does not include new cables, chain, or anything
tune-up is just that... adjustments, lube, and some grease. I don't take apart headsets, hubs, and regrease them, that would be a re-build.
for a $65 tune-up, i do a pretty darn thorough job, and inspect the bike to a great degree, and note it on the ticket.

the best way is to do as much assessment as possible when the customer is there bringing it in. You want to point out any major problems before it goes on the stand, and uses up too much time.

the thing that I really do not like, is when someone brings in a bike that is a cheap bike to begin with, they ask to have it worked on, you give them the ticket, with the job description, when it will be ready, you finish the job, call them, tell them how much it costs, they show up, look at the bike, and say, "well, it doesn't look that much different, you can keep it."

i can not tell you how many times a department store type of bike has been brought in, they ask to fix all these things, and then just ditch the bike at the shop. sometimes they don't even come back to pick it up.

i have no idea how the manager deals with this kind of stuff.
it always aggravates me. when i think of the hour i spent on the bike, and maybe they asked for new cables, housing, etc... and there the bike sits. unclaimed.

sometimes you can get a feel of this kind of stuff. (intuitive process)

Customer Service...
its a fine art
so dont feel too bad
depending on how busy your shop is, just keep knocking those tune-ups out.
i can usually do a tune-up in about an hour.
so in an 8hr day, i can get about 6 bikes done. some require more work than others.

personally i have a bunch of road bike buddies that let me wrench on their bikes, which is often just a quick tweak, of something. as crazy as it is, often i can do this after a ride, while we sit at coffee, and i just knock it out with a multi tool, and they give me a couple of bucks.

however, thats not Official legit business

its a fine line
for things that are more than a simple turn of a barrel adjuster, they take the bikes to the shop, where maybe I get called to work or not.