To accept donations through our shopping cart, you would have to add a set amount for your customers to donate.
Alternatively, you can use PayPal to accept donations, as they have a "Donations" button where you can insert the code into your site.
Login to PayPal and go to your 'Merchant Tools' tab, there you will find 'Donations' and there you can enter the information for your donations. Leave the amount field blank if you want donors to enter their own amount. Then when you get the HTML code, you copy it and enter it into your site.