In our Shopify online store, we used the checkout "tip" functionality to provide customers the ability to add a donation to their order. We used the "Theme Content" option to accomplish this and it works well. You can see screenshots of how we modified the language and the resulting section on the online checkout.
We also have retail stores that use Shopify POS terminals. We have the new POS Go Docks and are wanting to do a similar thing with the POS tipping functionality. We would like to either override the default "Tip" language on the POS Go Dock similar to the capabilities for the online store, or have support for an official donation feature.
The online donation option has signficantly increased our total sales number, which in turn increases revenue to Shopify by way of credit card transaction fees. Allowing the same capabilities at the POS terminal would benefit both us and Shopify.