Decoration and building limit??

Can someone please explain to me why I can only purchase four decorations at a time?? I am lv 31 and trying to decorate my zoo but it makes it kind of difficult when I can only buy four of one item! After I buy four of a decoration, tree or building it is no longer available. It just dissappears! Do you get more the more you level? If not this is a severe design flaw! Is anyone else having this same issue?

Tapzooadict13579 about Tap Zoo