The applicant shall post a letter of credit or a bond with the Town Treasurer sufficient to cover the costs of any required access, drainage, or safety improvements, the cost of regrading disturbed areas, covering with topsoil and seeding, and generally to conform to the provisions of the permit issued herein. The applicant shall present an estimate of such costs prepared by a registered professional engineer, and the Commission shall determine the amount of the bond or the letter of credit. The bond or the letter of credit shall be in a form approved by the Planning and Zoning Commission and from an institution acceptable to the Planning and Zoning Commission. The bond or letter of credit shall remain in effect through the period of the permit and any subsequent renewal, plus one year. The Town may require, at reasonable times throughout the life of the permit, proof of the continued validity and enforceability, as well as amount, of said letter of credit or bond. Failure of the permittee to produce evidence that the bond or letter of credit is in effect for the amount required shall be grounds for revocation of the permit. The Commission may reduce the letter of credit or bond requirements as portions of the work are completed.