Hello. When I asked a similar question, my friend suggested an app called Pro:Tagger. It helps me keep track of my customers and products. It can automatically add or remove tags based on what they bought or their history. Here is their shopify page:
https://apps.shopify.com/product-tags-manager. The main thing for me is that this app is easy to use and has a tool that allows me to tag many things at the same time. The best part is that it's not expensive, only $9 per month. I am very pleased with this.
