Just looked at your listings and, at the moment, you haven't actually taken out a shop subscription. You're just paying per listing which is costing you more in listing fees so having a shop subscription will actually save you money. To select your shop level and subscribe you need to go here: Choose your subscription level. First, have a look at the fee structure for the different levels here: Fees for business sellers | eBay to decide which is best. Once you hit 750 monthly fixed price listings, Featured is more cost effective.
Once you've got your shop subscription, clicking on your own username will take you to your shopfront. On that page, there is a "Manage my shop" link in the top right corner. Click on that and you'll see Shop Categories in the left panel. Click on that to add and edit them.
