You should try to draw these four seens to see if you are able and how you would draw these two characters :)
I like this idea Chrissie although instead of having the postbox jubilant right at the end perhaps it is jubilant to begin with then the dog pees, making it frustrated. As soon as it eats the dog it goes back to being jubilant again.
hey Chrissie, I agree with Ollie too and maybe you could have the dog peeing on the postbox, box gets frustrated, dog teases postbox, like do a raspberry, running around it in circles, sweeping dirt at the postbox or even pooping on it just to get the postbox really angry so that it eats it up and becomes jubilant? ^_^