If you have more than one user logged in under the same angler number at the same time (trying to guide someone step by step from a different computer using same angler number), it will say "pending" if you go back to try to change something, at which point you won't be able to erase anything. You also won't be able to finish the payment, basically you'll be stuck. And if you go back to re-select basic license or conservation charge, it won't be available anymore in the dropdown menu. Both users will have to log out, and then only one person would have to re-log in and you should have everything back at zero and then reselect whatever you are getting again.
If you don't backtrack at any point and click "pay" while having two people logged in with same angler number, you will get an alert window that essentially says that charges or licenses "overlap" as it senses a double payment for one same thing.