Commerce and influence in the world of Slower Than Light seem neigh-impossible at first glance. Shipping goods across interstellar distances is impractical for anything but the absolute most unique of artifacts. With the shortest trips between stars taking decades, and the extraordinary expense of safely moving humans between worlds, deploying armies would be utterly impractical; even if an ark carrying tens of thousands of troops was sent at fantastic expense, the destinations would have the time to develop a military-industrial complex almost from scratch before the attackers arrived to a defense custom-built to defend against them.
In Slower Than Light, the only ships to plow the void, with precious few exceptions, will be the unmanned probes gathering information on target starsystems, and the seedships carrying hundreds or thousands of colonists. How, then, can humanity’s scattered children have any impact on each other, enough to be a cohesive entity?
The only practically tradeable commodities are those that can be shipped at or near light-speed. The two major components of these will be Research and Power.
Research and technology is tradeable between worlds in Slower Than Light much as it is in other games in the 4X genre, albeit on a longer timescale. Colonies that can communicate with each other can share the fruits of their research programs, and give each other the information they need to replicate each other’s technology.
The other means of influencing other worlds is an outgrowth of technology. Any given star system will contain more raw resources than any colony is likely to use in the course of a game, and so the only limiting factor is if the energy available to harvest and convert those resources exists. Using extremely tightly confined beams of radiation, colonies can trade in energy itself.
Most obviously, newly-founded colonies will benefit from receiving beamed power from the homeworld. As the technology evolves, more power generation capacity and less loss in transfer will allow older, more mature settlements to give energy to the factions they want to help on other worlds, and help them maintain their influence over their colonies. As those colonies grow and generate their own surpluses, they then can send power on to other colonies, and the web of influence grows.
These two forms of trade interact in very different ways. Trading information is very straight-forward; presuming that each side actually wants what the other is offering, an exchange can be anything that both sides deem fair, although obviously lengthy back-and-forth negotiating will be exactly that.
Power transmission is a bit trickier. Every beam spreads as it travels, and so falls off exponentially as it travels further. Using ever-shorter frequencies of light, wider and therefore less divergent beams of power will help mitigate the power loss in transmission, but it is much more efficient to beam the power short distances rather than long distances.
The influence of power transmission is also dependent on where the power is being sent. There’s no benefit in beaming energy to a location if the power of the beam is less than they can get from their own star. Because of this, beamed power garners the most impact when used to influence settlements far from stars and other readily-accessible energy sources. Planets far from their parent stars and ships plying the interstellar void would be particularly dependent on the energy sent to them from other places, while planets orbiting close to their parent stars or those around particularly bright stars would be less moved by the offer of transmitted power.
Of course shipping very small objects on very faster courier will be an option, and might be necessary for certain objectives, but the real economy of the interstellar empire in Slower Than Light is built off of the faster courier there are or ever will be: photons.