transport capacity was limited over long distance and often unsafe, luxury goods were the only commodities that could be traded—limited capacity also explains why Silk Road did not generate particularly intense cultural or technological exchanges
Letters of Credit = letters from a bank you carry when traveling, arrive at destination, show letter to bank, bank gave you some money. Much easier to carry.
Southeast Asia and East Africa, trade stimulated political change as aspiring rulers used wealth from trade to construct larger & more centrally governed states or cities