First thing to note with this achievement is that the description is slightly misleading. You don't actually need to do any Merchant voyages to earn this achievement. The commendation in question is simply for selling loot items to the Merchant guild. The merchant guild accepts a variety of items including caged animals, cases of sugar, tea & spice (found on shipwrecks, washed up on islands or as Skull Fort loot) and gunpowder barrels.
Every time you sell any of these items the gold is added to your tally that can be found by pressing
, scrolling right to the merchant guild and looking at the second item on the top line. When this value reaches 150,000 you will earn the commendation and you can now buy the title "Gilded Merchant" from the clothes shop.
To earn this achievement quickly you can complete lots of skull forts as they usually reward you with multiple high value crates of exquisite spice.
The alternative method is to complete lots of Merchant Voyages. This involves purchasing a voyage from the guild, activating the voyage on your ship (do this while at an outpost), collecting the required cages from the merchant then heading to a relevant island to capture the animals on your "shopping list" and deliver them to the target outpost. It sounds like a lot of work but by using an animal location map maps.seaofthieves.rarethief.com/ you can cut out a lot of uncertainty. When you activate the voyage have a quick look through the list, if there are no black or gold animals (high value) then cancel the mission and start another; it's really not worth the bother of catching white chickens once you've leveled up past level 10.
There are a few boosting methods which involve rounding up loads of high value animals on a floating zoo before you take on voyages but this leaves you open to losing the lot if attacked by other pirates. Rare are also constantly patching the game to make these methods less and less effective.
Note. I unlocked the commendation about 6 days before I got the achievement. It actually unlocked when I sold a crate of exquisite spice that I found on a ship wreck. I had probably sold about 156,000g of loot to the merchants by this time.