Without the correct permission "View any Sponsor product" the sponsor will not be able to use the order link.
It would make sense to automatically give permission to anyone included in the Sponsor contact field as noted by the TODO in the code. So I created a patch that will allow adding the Sponsor product to the cart if...
1. The current user is in the Sponsor contact field
2. The current user is the author of the Sponsor node
3. If the current user has the View any Sponsor product
The patch includes one other small change to direct the user to the cart instead of checkout since this was very confusing. Now when they get taken to the cart they can remove items immediately instead of following the obscure link in the message. A user could have an extra ticket item or even an extra sponsorship item.