I have just been looking into this. When do you guys use them? Things that are limited to a country like the USA, things that are much more expensive outside the USA and those who charge a min postal fee like $60US even for a small item?
YouPost for packages, ShipItTo has more options (eg does mail as well as packages) but is more expensive. Are you talking about mail only?
Mainly parcels. I see that ShipitTo requires a monthly fee to use Oregon so it is taxed free within the USA. I am just looking at a medium format film camera from the USA. Since they only charge $48US to deliver a complete kit or as low as $38US body it leads me to think it's only for B&H etc who charge a min of $40US for a lens cap or that little 50mm lens or things like sunglasses which are marked up in NZ.
Use NZPost's YouShop. It's the cheapest option, it's quite reliable, and it's moderate speed. If you really need things fast use something else.