They don't want to buy diseased left overs, or road kill. They want to buy food for their dogs which is fit for human consumption. There is certainly some question about whether supplements should or should not be added to the diet. Personally, I feed little if any supplements. However, I appear to be in the minority.
Many people do feed supplements.
However, I feel that some suppliers are going well overboard in the amount of supplements they are encouraging people to feed. And there are some of these supplements that can and do cause allergic reactions. The last thing you want as a supplier is to make animals ill. My suggestion would be to put the supplements in a little sachet, with the ingredients fully listed, which then gives the customer the choice of whether or not to add them. Alternatively, you could market supplements in a bottle that your customers could then buy separately and add as required.