I've certainly done it both ways. Pushing a shipment back or outright canceling it. I've also added a whole bunch of additional things. In the long run, they still get most of my pet food/toy money.
I’m really reluctant to have a subscription for items where you can’t exactly predict the usage - it’s either too little or accumulates. Having the option to skip deliveries is a must.
You get an email and you remember you wanted to add something, and you go ahead and buy it.