Hey there Natalie!
Justin here from the Shopify Support Team! Thanks for reaching out to us today!
There is actually a Donations App available on our App Store, it is called 'Share the Love' and you can find it here: https://apps.shopify.com/share-the-love
If that doesn't work for your store you could always add the Donation as few products, in different denominations and create it on a Page/Collection and then use this App, so that your customers can receive tax receipts: https://apps.shopify.com/tax-receipts
Let me know if you have any other questions at all !
Justin | Shopify Support Guru
I don't know if you're still looking but we've developed an extension that sounds exactly like what you're looking for and is really easy to integrate and set up.
It also works as a solution to increase customer conversion and retention and is a fun way for your shoppers to interact with your brand.
Have a look and let us know what you think.
All the best