Cal here from Shopify, thanks for reaching out to us. The easiest way to achieve this sort of discount label would be to install a third-party application from our App Store. There are some great examples of discount labels: Discount Labelz, Product Label, amongst others. You can take a look at some of the options here.
Alternatively, you could hire a coder to custom code some labels for you. You may find a coder to hire over on our Experts Marketplace.