UltimateBackpacks is a premium plugin sold on Spigot. You can purchase it here for $8.

UltimateBackpacks is a fully customizable backpacks plugin. Backpacks are chests that can be right clicked from the hotbar to be opened. This plugin could be useful for factions, prison, survival and many other gamemodes.

All messages in the plugin can be modified and different types of backpacks with different names and capacities can be added. The plugin also includes a full API for developers.

Video Demonstration

You can purchase a copy of UltimateBackpacks through Spigot. Currently the plugin costs $8. Click this link to go to the page. From there either login or make a SpigotMC account and you will then be able to purchase the resource over PayPal.

Other Links
You can submit a feature request or bug report here.
You can view the full javadocs for the developer API here.