In practice - who knows, it depends on ordinary Japanese taking up the offer to actually use their bitcoin for something. Hopefully the 3% discount will encourage them, though some may decide they'll make more by just hoarding their bitcoin.
They actually get a 6% effective discount. 3% off for paying with BTC, plus a 3% rebate paid to their Coincheck wallet. Pretty huge! They also seem to have a similar program for electricity bills.
I wish we had deals like this in the US. AT&T (huge US telecom provider) added the option to pay via Bitpay but they offer a 0% discount.