You're indeed getting an up to 5% discount for using your credit card, but it's not just credit cards: any type of non-cash payment (キャッシュレス) can be used to get the "cashless reduction" (キャッシュレス還元), not just at Lawson but at any participant in the Cashless program. The discount varies from store to store, but at Lawson it's 2%:
Note that this is not just contactless/tap to pay in the sense of Visa/MC PayWave/PayPass, but regular credit cards, debit cards, electronic payments like Suica/Pasmo, basically anything at all that is not cold hard yen. Here's Lawson's promotion page on the topic (in Japanese only). At time of writing, the promotion is set to run until June 2020.
Also note that this is entirely separate from consumption tax (消費税), which is listed and charged separately in your receipt, so It may be possible to find a store which lets you buy tax-free and get the Cashless discount at the same time as well.