Hi K.,
In a way, it seems you've backed yourself into a corner by agreeing to do it as a favor the first time.
We ask our neighbor to "watch" our house when we are out of town, but we feel awkward asking for anything more. We stop our mail and the newspaper, and take our pets to the kennel. They are an active couple, and they ask favors of us only occasionally (like borrowing our tractor).
My point is this: we really like our neighbors, and would rather be more help and less burden. They do tell us "no" on occasion when we ask a favor, and we always give them the opportunity to say no.
If it's a true friendship, she should always have the right to *ask* and you should always have the right to say *no*, and neither one of you should get angry at the other for doing so.
(Maybe she doesn't know how easy it is to hold the mail, get an automatic plant-waterer, and install a cat door???)
T