If any seller shall fail to pay any utilities service taxes collected or assumed by him within ten days after he shall be required to pay the taxes to the city, he shall be liable to and shall pay in addition to the tax a penalty equal to one percent per day for each day the payment shall be in default after the first ten days. If any seller shall be in default for more than ten days, the city treasurer shall be authorized and empowered and he is directed to certify the fact of such default to the city attorney, and thereupon the city attorney shall be directed to enforce payment from such seller of the amount of such tax due to the city by action at law or suit in equity. If the tax shall be collected by action or suit, the seller shall pay, as an additional penalty, such reasonable attorney's fees as may be fixed by the court in which such action or suit is brought to reimburse the city for the expenses incurred for the services of the city attorney in enforcing collection; provided, however, that no seller shall be liable to the city for payment of any tax upon any uncollected bills.