If your submission is being sent by USPS Priority Mail Flat Rate, you must use a USPS Priority Mail Flat Rate Envelope (pictured below). These can be ordered online for free directly from the US Postal Service. 


Priority Mail Flat Rate Envelope


You can place an order for these free envelopes by going here


DIRECT LINK: 

https://store.usps.com/store/browse/uspsProductDetailMultiSkuDropDown.jsp?categoryNavIds=shipping-supplies%3apriority-mail&categoryNav=false&navAction=push&navCount=0&atg.multisite.remap=false&categoryId=priority-mail&productId=P_EP_14_F