Shipping Envelopes pack of 100 poly bubble mailers to ship USPS 7"X11.5" #4 Our poly bubble mailers are lightweight and durable, allowing for a great, safe, and cost efficient shipping method. The self-seal closure is tamper-evident and a secure way to close packaging. The bubble layer allows for overall protection of items within the package. This packaging method is also water-resistant and made of a multi-layer blend of polyethylene film.