Technical issues happen
Even the best of payment companies suffer technical issues, though it rarely happens for more than a short period of time.
There have been issues in the past with card networks (e.g. Visa) and acquirers (e.g. Nets) causing temporary crises where supermarket customers had to get cash from an ATM, come back later or give up their purchase.
It’s not just the card processor that can experience issues. Internet or WiFi outages happen locally. Card payments cannot go through without an online connection, but a card machine with a SIM card and strong enough GPRS/3G/4G can be your saviour.
Even so, parts of the country have weak mobile connections, rendering many card readers useless. With some terminals, you can process payments offline as long as it reconnects with the internet within a day or so to complete the transaction.
What else can you do if the internet drops out? With a mobile phone, you can send payment links that customers can open to complete the transaction online in their phone or computer browser.