You can use USPS certified mail as many times as you need. There is no limit to the amount of mail that you can send through this service. If you frequently mail letters or packages that require proof of delivery, then you can use this service over and over again. In that case, it might be wise to purchase a postage meter so that you can get the metered rate for postage. This could save you a decent amount of money on postage if you mail a high volume of items.