If a package contains anything other than documents, a customs form must be filled out.
B4 is a large letter size if the thickness is under 2cm and weighs under 500gm, so the best option to choose on eBay is the standard flat rate international post, use 'Choose Custom Location' then select New Zealand when specifying cost so that you can either only post to NZ, or to be able to vary the P&H cost to other, more expensive destinations.
