The QR code payment service PayPay is currently running a campaign where new registrations get a 500-yen balance.
The campaign runs until October 26 (per the official site), so hurry while you can.
(Added 2019/8/14) The campaign has ended, but as of writing this article, new registrations still get 500 yen charged to their balance.
What kind of service is PayPay?

It's a service provided by PayPay Corporation that lets you pay via QR code or barcode. Registration is free, and there are no usage fees. If you already have a Yahoo! JAPAN ID, you can sign up right away. The number of stores accepting PayPay is steadily expanding (per official site), and FamilyMart will also start rolling it out at select stores from November 2018. (FamilyMart official site)

As of 8/14, it can be used at a wide range of stores including restaurants, 7-Eleven, Lawson, and UNIQLO.
Let's sign up for PayPay!
- From the PayPay official site, tap the app link and download it.
- Launch the app, enter your information (or log in with your Yahoo! JAPAN ID), and complete the SMS verification.
- Once setup is complete, 500 yen worth of balance will be added to your account.

Get 3% back on purchases

When you pay with PayPay, 3% of the amount spent comes back to your PayPay balance later. Furthermore, at the time of writing, it appears there's a campaign at participating stores from August 1 to 31, 2019, between 11:00 and 14:00, where you can get 10% back. (See the official site for details.)
Bonus: Member stores get nice perks too

For stores (member shops) that adopt PayPay payment, the initial setup fee and transaction fees are both zero — a level of generosity that almost makes you blurt out, "Wait, you're not charging for that?!"
This should make it easy for the store side to introduce PayPay payment too.
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