You must have your Mercantile level at 75. When you speak to a shopkeeper, you have the option to invest to their shop. This will remove 500 gold, but it also increases the shops gold amount by 500. If your Mercantile level is 100, every shop will have 500 MORE gold, so in total, 100 skill + invest means that for example: a shop with 1000 gold will now have 2000 gold.