Hi, jan1304.
I'm a Japanese working in Tokyo for over a decade.
Let me answer to your questions one by one.
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A: i noticed that several providers have different prices for pupuru wifi with softbanks.
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As you have noticed, there are several providers having different prices, depending on device models, 3 major careers (NTT Docomo, SoftBank, au), and other options (ex. insurance).
Although you might have noticed as well, there are no "PUPURU WiFi" plans through the industry. PUPURU is just one of providers.
You can say that their service is fundamentally the same because they only borrow bandwidths covered by the major careers.
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I check all the same, insurance the same and so on...still i see a big difference. How can that be that pupuru official website is so expensive ?
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Although I don't know why PUPURU is set their price that high, I can introduce you a (almost) comprehensive list of pocket WiFi router plans in Japan so that you can compare each other.
Rental Pocket Wifi (Portable Wifi) Service in Japan
https://prepaidsim.visitjapanplaces.com/wifi----------------
B: is not possible to use google maps offline in Japan ???
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I've never heard of that you cannot use Google MAP offline so far.
You might need some instruction by Google?
FYI, take this.
Download areas and navigate offline
https://support.google.com/maps/answer/6291838?co=GENIE.Platform%3DAndroid&hl=en
Hope you'll have a great experience in your Japan travel.