SEALING TIPS:

The easiest way I found to close a milk carton mailer is to use a clear piece of packaging tape for the bottom.  The clear tape is really not noticeable! 
For the top, the strongest and quickest way is to put two staples in the top peak.  If you don't like the look of the staples, make an return address label or logo sticker and cover the staples.  That hides the staples, and keeps you in good standing with your postal carrier...they won't get pricked with the staples!

Hot glue is virtually invisible,however, it is a bit time consuming.  You will have to apply the hot glue and pinch the carton together until the hot glue is cold.  Once you glue the carton shut, give it a slight tug....if you can't tug the top open, than it is safe for the mail. 

There are no additional shipping charges for the milk cartons.  They are billed just like a typical box.  They are better than a box, because they are lighter (cheaper shipping charges) and waterproof!