Hmmm... is this a new skill on everyone's sheet or is this a feat someone can take. If a skill, wouldn't it be associated with a specific attribute, i.e. Intelligence. So you would call for an Intelligence (Commerce) check?
Seems to make more sense as a feat. Anyone with this feat can add their proficiency bonus to any commerce-related skill check, such as check called for when appraising items or bargaining to buy or sell something. But it seems like most players wouldn't want to spend a feat on it.
Instead, I would probably make it a feature of a Background. Someone with a "Merchant" or "Fence" background can use this "Commerce" ability.
How, exactly, is it used in your game? What does your skill list look like and how does it operate compared to the standard 5e skill list?
I already have and always will use the variant rule that does not assign attribute to skill. So I re-did my skill list for all players that got rid of some skills that are unnecessary for this variant rule use, and added in new ones that had space available. So for instance... Acrobatics was removed from my list, and players instead make Dexterity (Athletics) checks. Sleight of Hand was removed and players instead make Dexterity (Deception) checks. Intimidation was removed and players instead make Strength (Persuasion) checks.
Culling my skill list of the little-used skills allowed me to add in new ones for subjects and categories of things that I feel the game benefits from. One skill about handling money (Commerce) is one. A skill about the nobility and high-society (Etiquette) is another. And one about people and actions in low-society or the common folk (Folklore) is another. Then because I don't assign ability scores... how a PC interacts within that category determines what the pairing is. I tell the player which ability score is being used... they tell me if they have a skill that is applicable and how what they are doing makes the skill applicable. And sometimes I might allow for multiple skills to possibly apply.
Remembering the standard of a specific nation would be an INT check, and they could then apply Etiquette. Trying to convince a duke of something would be a CHA check, and based upon what the player does or says they might be able to apply Persuasion or Etiquette to it. Figuring out that the king's staff wizard isn't who he says he is would be a WIS check, and maybe they could apply Insight, or Arcana, or Etiquette.
I don't use feats for these things, because feats don't show up often enough to warrant these kinds of character-defining traits. Plus, no one in my games uses feats for characterization defining things, as they usually are already playing their characterizations and don't need a feat to exemplify it. If a PC is an actor, they will have already been playing as an actor and don't need to wait until 4th level to take a feat that shows it off. Likewise, if someone is good with money they aren't going to wait until 4th level to take some sort of Commerce feat. Instead, I make fiscal use a skill that allows anyone who wants it to take it from the very beginning. And obviously if you have a Background that is focused on it (say Guild Merchant), then it is an option to select when you take the Background.
For me... using money is too important a part of adventuring to not have it have a more important place in the game. And the skill list allows that importance to have a place.
