You can easily send first class mail parcels using Sonlet shipping labels.  You'll need to weigh the item inside the envelope and enter the weight into the input area to calculate the correct postage. The USPS allows you to send First Class parcels only for packages that way less than 1 pound (15.99 ounces). You cannot reuse priority mail packaging, such as a priority envelopes or padded envelopes.  If you do, the USPS may return the package with insufficient postage (you will not be refunded for the postage you already paid) or they will charge your customer additional postage upon delivery.