I think the point many people are making in here (and it not only for this particular shop but any shop) is:
1) You are the customer, you have the purchasing power - you are holding the money and shops wants your money. Only way s/he is going to get it is when you agree to give it to them. Say Yes when you intend to buy, no if not.
2) If you lack the knowledge when they offer to change something or fix something, call someone else for advice. They are entitled to make all the sales pitch in the world, you are entitled to decline. There is no force here. Don't cry victim if you agreed, and later only find out you taken for a ride.
3) Don't go mamak, next door for a smoke or MIA unless you gave them clear strict instructions that anything extra that was not agreed to should NOT be put on without your permission. Leave your handphone no. if he really needs to contact you. Tell them to wait till you get back to the shop to decide, don't decide over the phone.
4) Unless you really didn't agree to any thing, you come back and you got a bill for things you already said no or charged a price that is not following the sign board / price tag, there is nothing much the consumer tribunal can do or will do for you.
Anyway, thanks for sharing with us your experience, it can happen to anyone and at any shop. You've learnt a very valuable lesson (though u frustrated) and it will come in handy next time. Imagine if you haven't learn it at a tire shop? You might have gone to a workshop to change a ste of belts and come out with almost a new engine. The advise shared by the others are all valid for any scenario..
cheers buddy!