An Incorporation need to issue a tax invoice electronically(전자세금계산서) according to the Korean tax law, so need a digital certificate(for issuing tax invoice;세금계산서용 공인인증서) without exception.
For sole-proprietors, need to issue electronically if it's previous years sales revenue was more than 300mil.won.
Once it's violated, need to pay a penalty tax(가산세), 2% of the amount missed.
You can issued a tax invoice by the 10th of the following month, for example you can issue a tax invoice by the 10th of July for June invoice amount without fine or penalty.
If you needed to issue it electronically but issued a tax invoice in manual(종이세금계산서), you need to pay 1% of penalty, if it's issued lately 1% of penalty also.
If you missed to issue a tax invoice, need to pay 2% of penalty.
You can download all of your tax invoice(sales and purchase) at the NTS server in excel format whenever and wherever you want, so you may not need to print those electronic tax invoice at all.
But you need to collect a manual tax invoice physically and need to keep it more than 5 years with other commercial documents. And also need to check the eligibility of sole-proprietorship since some of them close their business registration while issuing a manual tax invoice and runaway, then you need to pay those VAT as well as some penalty.
So if possible, please ask to issue all of tax invoice electronically if it's more than 1mil.won without exception to avoid any dispute with the tax office.
More details, please send me an email to firstname.lastname@example.org, gooday!