Bubble mailers can be considered either Letter mail or Flat mail based on the size of the envelope, the thickness, and the weight. Letter Mail must be less than 6" x 11.5" and can not be more than 1/4" thick. As long as an envelope is within these specifications and weighs less than 4 ounces, it is considered machineable Letter mail which means it can be mailed and tracked via LetterTrack. Flat mail or Large Envelopes(these terms are used interchangeably) have a maximum size limit of 12" x 15" and can not be more than 3/4" thick. As long as a mailpiece is within these specifications, it is considered machineable Flat mail which means it can be mailed and tracked via LetterTrack. Our proprietary software allows you to print on envelopes of various formats or address labels in various sizes and adds a USPS intelligent mail barcode above the address block of the recipient of your mailpiece. As your bubble mailers travel through the postal system to its destination, the intelligent mail barcodes on your mailpieces are scanned at each facility. LetterTrack, captures these scans to provide the seller and buyer with real time tracking emails each time a mailpiece is scanned. When a mailpiece reaches the destination post office to be placed on a USPS postal truck for delivery, the mailpiece is marked as Out for Delivery. Since the USPS has now geo-fenced most of the mailboxes in the US and has also equipped their mail carriers with geo-enabled devices, the final tracking event for most envelopes will be Delivered in Mailbox. This tracking event indicates the exact date/time that a postal truck containing an envelope with an Informed Visibility barcode for a specific address has traveled past the mailbox for that address. Your mail will reach the recipient in 3 to 5 business days since it is being sent via First Class mail.