Idea:

When a user cancels there service on the Blesta panel, a feature should be added, where it would automatically add +1 to the availability of the product cancelled.

This would enable more convenience for hosting owners to quickly re-assign stock availability.

How i believe this should be implemented:
You should be able to enable or disable this feature for each product, either via the "New Package" when you create a package, or via the "Order Form" for that package.