The first thing you need to do is to finish Myriam's Homestead Missions. Now you need to have 3 Land Convoys constructed AND have purchased the extra slots to carry more items in the convoys.
Make sure all the Forts are Liberated and all the districts Liberated in New York.
Now head to your ledger and purchase the following supplies from Myriam:
Bear Pelts x10 - Total Cost £2400
Beaver Pelts x10 - Total Cost £1600
Deer Pelts x10 - Total Cost £1200
Total Investment - £5200
Now head to your convoys and load up the first convoy with 9 Bear Pelts going to Rhinelander's Sugar in New York.
Now load the second with your last Bear Pelt and 8 Beaver Pelts,
and finally your final convoy with the last 2 Beaver Pelts and 7 Deer Pelts.
You may have more of any of these pelts simply from your animal hunting in the Frontier, but lets assume for this example you only have what's purchased.
You will receive the following return on your convoys:
Convoy 1 - £11,232
Convoy 2 - £7,904
Convoy 3 - £6032
Total Profit after initial Investment: +£19,968
You can repeat this every 15 minutes or so, from any ledger at any General Store to get enough money to buy anything you could possibly need in the game.