While it is not specifically useful, I used to do that a lot. At that time Small Packet included insurance, which is no longer true.
However, in the interest of selling the sizzle and not the steak, providing this invoice number reassures your customer that you actually did buy postage and therefore posted his parcel. If he has other problems (damage or delay) he would be bringing it to you for resolution. Neither tracking nor insurance provides any benefit to the buyer.