Here's another option to consider!

In addition to (or instead of) having donations in the purchase path of a ticket event, you may add them to the checkout page.  To accomplish this, begin in your event's Dashboard; then select Donations >> Donation Setup >> Checkout Add-On. 

This feature will allow you to define up to three different dollar amounts:

When the purchaser arrives at the checkout cart, the donation amount(s) you've defined will appear beneath the section requiring payment info:

This "last minute" prompt is an effective way to collect smaller donations and provides a great alternative when you've chosen not to have donations appear along the ticket purchase path.