rakuzas, what OS are you using? I had that problem when I was using Windows Vista. You can try changing the compatibility mode to Windows XP (Service Pack 3) and tick Run this program as an administrator.
I don't know if this will work for you but Euphy's Multi-Shop script can do that.
setarray .Currency[1],512,513,513,513,513,513,513,513;
setarray .Shop1[0],513,5,1229,20;
1 Banana will cost 5 Apples.