My idea, I'm not sure is exactly possible. It would be a auto feeder, and here it how it would work.

1. It would scan for food every so often. (Maybe a setting that is changeable)
2. It would notice when there is no food in its 'protected' bowl. (Perhaps it would only search for empty bowls... saving people having to buy more then one feeder)
3. You would pay into the auto feeder, thus making it able to only spend what you put into it
4. It would buy the item. Either sending it to you - thus giving you notice that a bowl is empty- or rezing it beside itself.

Settings I think would be needed/able to be changed:
1. Scan range
2. What type of food
3. How much of that food
4. (Almost forgot to add) Scan delay.