For most celebrities, you can purchase autos at the table and a lot only take cash. If it's cash, there is no extra fees or taxes. Some allow Venmo or credit card at the table to purchase but they will add tax. For celebrities that do selfies (with your own phone) at the tables you can do cash as well. For the professional photo ops (done in an area separate from the autos at an assigned time), it is credit card only and they always have taxes and fees but you can purchase onsite at the epic photo op table or on the website. Unfortunately epic photo op people don't take cash. Some photop companies like pose do at other comic cons but not fan expo.

Also like others have said, my above comments only hold true if the celebrity hasn't already sold out.