Every invention developer shall maintain all records and correspondence relating to performance of each invention development service contract for a period of not less than three years after expiration of the term of each such contract or completion of all obligations pursuant to the invention development service contract, whichever is later. The records and correspondence required to be maintained by this section shall be made available to the customer or his representative for review and copying at the customer’s expense on the invention developer’s premises during normal business hours upon seven days’ written notice, such time period to begin from the date the notice is placed in the United States mail, properly addressed with first-class postage prepaid.