All goods are transported via trade portals. Goods get transported whenever trading on Merchants' Den or buying at the Trading Office.
Every trade portal has a maximum capacity of 25,000 items. That is NOT to say you can only send a maximum of 25,000 goods per portal. It rather means that the goods you send will be transported in batches of maximum 25,000 items. Transport time depends on the distance between the sender and the destination but also on the amount of goods sent (number of batches). However, transport time may also be subject to effects.
Trade portals require an upkeep when in use. The current base upkeep is 50 
per round. Portals require no upkeep when not in use.
The base cost to build a trade portal is 1000 .
All the info on trade portals can be found in the Transports subadvisor screen, located under the 
Commerce Adviser.