I have to agree that you shouldn't mention guarantees that are not only out of your control, but subject to things you can't know like buyer location. When you mention someone else's guarantee, without clarifying all the provisos, you're effectively endorsing it and will usually be the first one held responsible when it doesn't happen (be it your fault or not).

I would personally just have 'Express shipping available'. When people send me enquiries about shipping times and express, I let them know exactly when I can post the item (often enquiries come later in the afternoon, so it's not possible for me to post the same day), and that exact delivery time will depend on their location. I mention my postcode, and provide the link so they can check, but even if we're both in a location that should see next day delivery, I only ever use the word 'should', just in case AP don't follow-through. :8}

In these circumstances, I would actually accept responsibility just because the wording was a little ambiguous (if just to those who has no real knowledge of how Australia Post's express network works), clarify what was intended by mentioning the guarantee, as well as apologise for any confusion, misunderstanding and inconvenience. You may find that once you acknowledge that, the buyers will be more receptive and understanding, and may agree to a feedback revision request.