Cal here from Shopify, thanks for reaching out to us. It's great to hear you're getting so much traffic. In order to give the best advice, I'd need to know what type of advertising you are doing? Not all traffic is equal, and it's likely that you are not targeting the correct audience.
Have you given thought to who your customers are? What are their behaviors and interests? There is a great article here on crafting marketing personas that will get you thinking about your ideal audience.
Keywords would be niche-specific, and would be based on research done on what your customers are searching for most. For keywords, I always of what the customer is likely to be looking for. Many customers seek out answers to problems, which is why I strongly recommend creating a blog on your Shopify site and answer the problem questions within your niche! It's a great to attract organic traffic!