I strongly believe that 12 days (5 from the invoice + 7 from the NPB notify)
for a total unresponsive buyer is too much waiting for getting your items back
on your inventory.
I know that I can cancel the order myself, but that would feel simply wrong:
the user is a NPB, they must face the consequences, they cannot get away with
it. This way, they could do this again, to me or to another shop and bricklink
is mainly a community.
So, I would suggest that if the system sees that there are no messages FROM the
buyer to the seller, the buyer has 2 (instead of 7) days to reply before the
NPB is completed.
If, instead, there is a communication, 7 days feel right.