It's important to differentiate economics and politics for this question. Let's start with the economics.
Economics is the study of production. In human civilisation at least, everything takes some effort to produce. Sure, that effort may be as 'simple' as planting some seeds, watering and waiting, but someone still has to do that (and this is why we call farming a primary industry). What that means is that everything we produce in the form of goods (like food) and services (like banking) comes with a price because there is a finite supply AND a finite demand, the ratio of which drives the price.
In GuineaWorld, some of the supplies that arrive are going to meet or exceed demand, and some aren't. There are also going to be some finite resources that must be maintained, like the mall itself. Buildings require regular maintenance to stay standing and eventually the Mall will fail structurally if such work is not undertaken. In this, we see 3 types of goods in this economy (infinite, finite and irreplaceable) and there are also services that will need to be provided.
The infinite resources (those delivered that meet or exceed demand) are effectively free. No effort is expended to generate them, and there's no reason to hoard them or even store them.
Finite resources (those for which there is more demand than what is supplied by the trucks) will have a price, as there is some competition for them. That means you need money, but given that services like maintenance need to be provided, some inhabitants will work and therefore have money for the items in higher demand. These become controlled resources in effect, and will be traded between inhabitants meaning that there are also jobs for things like security in this world.
Irreplaceable resources (like the Mall itself) represent an asset that if lost, would cause the complete collapse of your society. In our world, this would be the equivalent of salt contamination in farm lands, or extreme climate change. In previous times, human civilisations have either fallen or migrated due to massive and sustained droughts, for instance.
In this case, your Guinea Pigs will need to ensure that maintenance of the mall takes place. In so doing, they are effectively creating their own version of an Environmental Protection Agency (EPA) to preserve the longevity of their society.
Now for Politics.
It's not necessary for the Guinea Pigs to run an authoritarian regime to protect this secret and if the vast majority of goods fall into the Infinite category, the effort may well not be worth the benefit. Ultimately, power struggles, wars, and the like are about controlling resources.
That said, if most of the resources are infinite, then your society is almost completely a leisured class, meaning that work is almost unnecessary. Anarchy can work as a political model if no-one's missing out in any way. The trouble with that is that the Guinea Pigs realise that the one irreplaceable asset (the mall itself) still needs maintenance, so some work still has to be done.
They don't want to do it, so they set up their government and religion to hide where the goods are coming from because that way, they can take credit for it. If their 'function' is to provide the goods, then they've done their bit and the rest of the society can contribute by doing the maintenance work. On balance it would look like a fair trade to non Guinea Pigs, who don't know that the Guinea Pigs aren't doing a thing; they're just letting the goods enter the economy.
The problem with basing an authoritarian regime on a secret is that when an entire class (or in this case species) knows the secret, it's going to come out eventually. The reason why terrorist groups operate in cells is because they know this fact. If 4 people know a secret, the chances of it coming out are roughly 50%. That means you keep your organisation separate from itself, organising yourself into small groups, where only a single member of each cell knows a single member of another cell.
All it would take is one disenfranchised Guinea Pig to let the cat out of the bag, so to speak. Sure, you can put in serious penalties for such treason, but once it's done you can't unring the bell.
The Guinea Pigs would be FAR better served allowing a contributory government model to form (those who work the hardest in maintaining the mall and performing other services get commensurate government positions) while they maintain a religion that pontificates about the source of the mysterious goods and setting themselves up as intermediaries between the populace and the 'Providers' as defined by the dogma they write. That way, they actually have to do LESS work than they would trying to maintain that authoritarian regime.
Of course, that leaves them open if the trucks stop coming one day because no one on the other end is maintaining the production facility.
