First, wait until you actually get the package back. Then see why is was undeliverable. The most common problem is something like it was an apartment or multi-tenant building and the buyer neglected to include the apartment or suite number.
The buyer will contact you soon enough. If they don't then contact them and explain what happened.
If it was a poor address that is the buyer's fault then you are within your rights to ask for an additional shipping charge to re-send it. Or issue a refund but deduct the shipping charges. IF it was the buyer's fault.
Beyond that there are too many variable to second guess them here. Come back here once the package is back in your hands and you have a definite cause..
"Laissez-faire capitalism (AKA The Great Material Continuum) is the only social system based on the recognition of individual rights and, therefore, the only system that bans force from social relationships." ~ Ayn Rand
