"Just tell them
that you picked up the wrong box by accident."


I don't know where you buy stuff, but in my experience, if the vendor
accepts returns, which most do, the boots not fitting are a perfectly
valid reason for returning them, without having to lie. I'd have to
disagree with foot beds being a solution he'll likely be happy with.
If a boot is oversize, foot beds may help, but are unlikely to fix the
problem, as foot beds only take up some room vertically, which may
reduce problems like heel slip. But they don't address the real
problem which is the boot is too long and the foot will move for and
aft.