What shipping bags are good for
Shipping bags and poly mailers are often used for clothing, fabric goods, soft accessories, and other items that do not need rigid box protection. They can be lighter than boxes and can resist normal moisture better than paper-based packaging.
When a bag is not enough
A bag may be the wrong choice for rigid, sharp-edged, fragile, crushable, or high-value products. In those cases, a box or rigid mailer may provide better protection and customer confidence.
Weather and presentation
Some sellers use an inner bag or product sleeve inside the mailer. That can improve moisture protection and presentation, but it adds cost and material use.