Interstellar Trade
Interstellar Travel is difficult and expensive.
Interstellar Trade is even more so.
There are already extreme costs associated with travel in terms of energy and time. A large amount of power needs to be used to travel between star systems. Most trips can take upward of 2 weeks, during which the crew still needs to be paid, and the FTL effects from the local SLE need to be sustained. In order to make these trips profitable, a massive amount of money needs to be generated per trip. This means that only very expensive for its weight materials are traded between star systems. This often includes the exotic materials used for FTL systems and cutting edge electronics. Raw materials like Iron, fusion fuels, etc are often so abundant in local star systems that trading them between star systems doesn't make economic sense.
The other major commodity transferred between star systems is information. Software, technology, and biological information like DNA sequences are all major industries. Similarly, entertainment and the rights to distribute have created a massive interstellar trade sector.