Poly mailers are a good investment for the business, or even for home use if you have a lot of things that you need to mail out that you must make sure will not be opened in transit.
While it is rare to lose things in the mail, it does happen. Millions of people year lose something in the mail for one reason or another. This can be due to being lost by mail carriers or just by being illegally opened.
You can generally find poly mailers in a wide variety of sizes. They make them in sizes that can fit standard documents up to full clothing items.
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This makes them popular during the holiday season because people like to mail the gifts that they buy out to the recipients in these types of mailers. While they are popular in the home during holiday seasons or birthdays, they are especially popular for businesses because they are so secure.
You can often find these at Mailer office supply stores for varying prices. Often they are much more expensive than a standard envelope, but the extra security they provide is more than worth it.
It's probably best for you to go to a local mail supply store and see what they have. It may be more expensive to do so, but there is no point in buying excessive amounts of envelopes when you know you will not use them.