Are you sure there was no tax before 1991?
This album from 1990 has two prices, one without tax:
http://vgmdb.net/db/covers.php?do=view&cover=64626
Note that the tax-inclusive price is the one displayed prominently on the spine.
The price listed for this album doesn't agree with the one on the back (of course, our price could be an error.)
http://vgmdb.net/album/3089