You can usually 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. Click on 'Details'.
Next, click on the 'Get Add-On' button (within orders comprising multiple tickets, there'll be one of these buttons for every ticket), and any upgrades or add-ons available to you will be shown there; select that which is of interest and follow the prompts thereafter. If you don't see an option to add an upgrade to your order, it means that the event you have tickets for doesn't offer upgrades or add-ons.