Section 9. A principal who wilfully or knowingly fails to comply with provisions relating to the prompt payment of commissions set forth in section eight shall be liable to the sales representative in a civil action for the principal amount of the commissions owed and for an additional sum up to three times the amount of commissions and for reasonable attorney's fees and court costs. A principal who is not a resident of the commonwealth and who enters into a contract subject to the provisions of sections seven to nine shall be deemed to be doing business in the commonwealth for purposes of the exercise of personal jurisdiction over such principal. No provision of sections seven to nine may be waived, whether by express waiver or by an attempt to make a contract or agreement subject to the laws of another jurisdiction. A waiver of any provision of sections seven to nine shall be void.