Many eCommerce companies find USPS services cost-effective and efficient. USPS is price-competitive for shipping small packages. Your eCommerce fulfillment operations might use Priority Mail, ground delivery, media mail, or first-class package service. Also, when you ship high-value items through USPS, you may want to pay for extra services, such as insurance.
However, you’re unlikely to need a USPS Certificate of Mailing for your order fulfillment. People more commonly use this service for official documents. You might want a Certificate of Mailing if you have to send a time-sensitive legal or financial document.