You have a few options for purchasing groceries:
You can buy groceries upon your arrival at the charter base or at nearby supermarkets such as Lidl, Kaufland, Konzum or Ribola.
You can order supplies online. When placing your order, be sure to specify the name of your boat and the marina you're departing from, as well as the desired delivery time. It's recommended not to schedule delivery too early; ideally, aim from 3:00 PM. You can order provisioning from Jam Yacht Supply or Supply at Sea, who can deliver directly to your yacht.