If things go wrong
When a service provider charges more than you expect, your next steps depend on whether a price was agreed before work began.
If you got a quote
Talk to the service provider. Politely point out that the final bill is higher than the quote. It's your right to refuse to pay the extra amount — unless you changed the scope of the job once the quote was confirmed.
If they insist on the higher price, you can take a case to the district court or Disputes Tribunal.
If you didn't get a quote
Ask the service provider to explain their final bill. If you got an estimate for a lower price, ask why the difference. Listen to any reasons they give, eg you chose more expensive paint.
If there's no good reason for the high price, check the going rate. One way to do this is to ask other service providers what they would charge for the same job.
If the going rate is much less than your final bill, try to negotiate a lower price. If the service provider won't negotiate, you can refuse to pay more than the going rate.
Either side can take a case to the district court or Disputes Tribunal.