When a constituent is attempting to buy tickets for an event, they may experience the following error:
Error: please enter zero or valid positive gift amount
This means that they deleted the 0 quantity from one of the ticket types. The system expects a number to be in each ticket type field, even if it's a 0.
The solution is for the constituent to enter a 0 if they don't want to purchase that ticket since the field cannot be blank.
Note: If the constituent is using Chrome, it's possible that they are using auto-complete which has been known to enter invalid characters in the Additional Gift field which can't have text characters and can't be blank (must be $0.00 or $0 if they don't want to donate an extra amount).