This popup is simply warning you that the selected envelope is larger than a regular size envelope and may contain a lot of pages inside which may utilize more of your page scan allotment than you normally would.
Not to worry though! Traveling Mailbox is looking out for you! If you request an item like this to be opened and scanned and it uses all of your allotted page scans for the month, we will simply drop those page scan counts, and charge a small flat fee of $2.50. This will avoid any costly overages for the month.
One quick note: After the open and scan is completed, in order for this charge to take place, the item must contain enough pages that pushes you over your limit for the month.