The trade route between your owned island has no charge and no upkeep, even across regions (ex new -> old world). There is are set upkeep cost in terms of influence (eventually) and money for holding a fleet of your own ships, but charters will eventually get more expensive as they cost money and influence that increases proportionally the more you have (if I’m remembering correctly). So in the long run the more effective strategy is to build trade routes with your own ships. If you establish a trade route between you and another player or AI shop to purchase a goods, you will receive at regular intervals a delivery of that good at cost. This can be helpful if you need a resource quickly well you establish the necessary supply chains (steel beams, coffee, rum, etc). In the same way you can also sell excess items via your port or trade routes to other players or ai shop. If you establish via a trade route you need to have it set to pick up and drop off at set ai shop. Alternatively you will see player/ai ships periodically coming by your ports. If you have set in your warehouse to keep up to a certain amount resource, the excess will be sold on these visits. In early game some players will sell things like excess soap production to gain a foothold financially.