Although we do not have Non-profit specific features or benefits that you'll receive, we have created our Sitebuilder, and especially our free package, with non-profit organizations in mind. So all the Sitebuilding features are available to you with the account option you are already using, including the option to add a Donate button via PayPal. You do not need to switch over to a different package in order to take advantage of this or any other feature.
We have a tutorial that explains how to create an online store using PayPal widget on Yola. Here is the link: Setting up the PayPal e-commerce Widgets. Section 2.3 in the tutorial will take you through the steps of adding a Donate button to your site.