Plaster is heavy, shipping will never be cheap. Which is why I bought big bags.
One of the Woodlands Scenic plasters is Herculite I believe. You can probably buy that locally and is pretty strong. Just bear in mind that it's a 'premium brand' which will affect costs and the packet isn't that big. Plenty to have a try with and see how it goes, if you decide to do a big project however, I would suggest getting a 25kg bag. Even with shipping it's going to be much cheaper and you get more options.