Ability to take Donations at time of purchase

When someone is shopping we should have the ability to take solicit a donation amount.

This should be charged along with the event being purchased. 

Ideally it would have separate tax rules.

It should be purchasable by anyone who can purchase an event.  For instance, the non-member parent of a Junior who is buying lessons or camps.

This is the ideal time to collect donations, when someone is spending, and it's just a matter of adding a little more on top.