In my experience (10 years living in Japan) it's actually extremely easy to do bank transfers, so easy that actually many people use bank transfer to pay for on

In my experience (10 years living in Japan) it's actually extremely easy to do bank transfers, so easy that actually many people use bank transfer to pay for online purchases. It does cost a bit though (from ~100 to ~600 Yens depending on the amount, time of day, and size of banks involved), and the sender has to pay that amount, not the receiver. That makes it an even bigger incentive for sellers to accept this way of payment.