How to add optional donation button

I’m using woocommerce for a business page.

I want to add an optional donation button to the checkout – either a tick-box or a drop down menu.

Is there a quick an easy way of doing this without paying for an additonal plugin? I thought of adding product variations and having a drop down box that way, but it will take me a long time to double up all the product variations for every product, so thats not ideal.

Any suggestions much appreciated.