Don't worry. You can add an add-on, be it parking, camping or other such things to your order even after you have purchased your tickets.
Simply log in to your Ticket Fairy account and locate your order in My Tickets.
Next, click on 'Get Add-On' and follow the steps. Any upgrades or add-ons available to you will be shown there.
If you do not see an option to add an upgrade to your order, it means that the event you have tickets for does not offer upgrades or add-ons.