If anyone is still looking to add tipping functionality to their stores (outside the checkout page) I have also created an app that does this!
The app is called Tip Quik and it is completely free to use.
When a customer clicks on the "Checkout" button on your cart page, it opens up a tip modal that prompts them if they want to leave a tip.
Some of the features of this app:
1. Works with all official Shopify free themes out of the box.
2. Ability to set your own tip percentages.
3. Edit the tip modal pop-up text and colors.
4. Let customers add a tip before reaching checkout.There is also a premium plan option that allows your customers to set a custom tip amount and gives you developer support to completely customize the tip modal functionality.You can check out the app on the Shopify App Store here: https://apps.shopify.com/tip-quik-1Demo store here: https://tip-quik-example.myshopify.com (password: tipquik)