I haven't dealt with this seller before but guess [like Klet does], that he doesn't accept Paypal, few Japanese do because they're not aware of Paypal's changes to policy in the last year or so.
It used to be tricky for Japanese users to accept Paypal payments because Yen wasn't an acceptable/available currency and I believe Japanese banks are strict on the sorts of transfers they allow.
I assume it's now much easier for those in Japan to use Paypal so more SHOULD use it. Word needs to get out about it.
Failing that, just send cash. Buying & mailing Yen won't cost you much- you might get a cheaper conversion rate than Paypal give anyway.
Just because we use the internet so much for our dealings nowadays, we shouldn't neglect the old skool snail mail! *I'm not a representative of the postal services, honestly

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