If the item isn't full-filled by Amazon (ie my stock isn't in an amazon warehouse) then I need their address details and phone number (for the courier) to ship the item.

Dealing with Amazon, as a seller, can be a terrible experience. Using their full-fillment service more so. They don't check returns so the next customer night get only part of a product, or in lots of cases a bogus product when a buyer fills the box with other cheap junk.