Would be good if people could start with 1 slot, and then purchase more slots for X scrap, ut to a maximum of X slots.

Currently there are some addon plugins that add this capability, by dynamically granting permissions, using the legacy row permissions. I think there are two, both named Backpack Upgrader.

Long term, I'm thinking about allowing for an alternate mode of the plugin where capacity is determined not by permissions directly, but by a capacity profile which is determined by permission. A capacity profile would have an initial capacity, which applies at the start of a wipe automatically, and it would have a max capacity. Other plugins would be able to raise the player's current capacity up to the max, via an API. That would allow for a combination of plugins to influence capacity simultaneously, such as play time rewards, skill perks, consumable items, and purchasable upgrades.

I could also add support directly into the plugin to purchase the upgrades, but I will need to do the above redesign first as a prerequisite. To set expectations, this feature will probably not come for at least a few months, because I have spent a lot of time on Backpacks in the past 6 months, and I am scheduled to work on other plugins for a while.

Some nice sounding ideas, look forward to one day using those!