PostSeal is described as 'Easy and quick way to send letters, forms and documents from your mobile phone, tablet or computer. No sign up required. Why send letters online? 1. You are abroad'. There are three alternatives to PostSeal, not only websites but also apps for Android. The best alternative is Mail A Letter. Other great sites and apps similar to PostSeal are Mailform (Paid) and SendOvernightMail.com (Paid).
Mailform is the easiest way to print and mail documents online. Automatically import documents from any source, including letters from Dropbox, form from Google Drive, and invoices from Quickbooks, Xero, Harvest, and more.