This community-run travel agency can help you organise your whole Kumano Kodō trek. You can book accommodation (and pay by credit card), store your luggage (¥500 per item per day) or have it shipped elsewhere on your route (price varies), pick up detailed maps, bus schedules and last-minute supplies, and have any questions addressed in fluent English.
Walk straight out of Kii-Tanabe Station, cross the main intersection and it's on your right.
The mark-up for its services is minimal compared to the support it offers. Its online booking system is highly recommended, and it's the only way to book many of the locally run minshuku (inns) along the trail. Also, by booking through Kumano your hike is registered with the authorities, meaning that they will be alerted if you don't show up at your planned lodging.