Has the contractor with whom you are doing business paid his assessments to the CSST? If he hasn’t, you could be held responsible for payment of these assessments as the employer who retained his services!
An employer who receives a notice of assessment or a request for information from the CSST regarding a contractor whose services he has retained should always attempt to obtain details from the CSST that will help him prepare his response and evaluate whether it is possible for him to contest the claim. A CSST notice of assessment can poke a large hole in an employer’s operating budget.