Sending domestic package at post office

I live in Japan and want to send a package (birthday gift) within Japan.
I already boxed, taped, and addressed the package using Daiso supplies.

However, I just read that boxes must NOT be taped ahead of time.
I will have to fill out a form with what is in the box.
The box must be open so the post office can look at what's in the box.

Is this true?
It'll be a hassle to remove all this tape and redo it.

Also is the form difficult to fill out?
I can't read or write Japanese.

Over the years I've shipped plenty of sealed boxes within Japanese from the Japanese post office. No problem whatsoever. If you get hassled, you could always go to Yamato or to a convenience store and use takkyubin. Yamato, for example, has a low-charge box rate. I used it recently to send a box within Japan for about 530 yen, I think. They'll even sell you a nice box if you need one for about 50 yen, and you don't need any extra tape since the box has adhesive on it already. That's cheaper than buying stuff at Daiso.

No, you don't need to keep the box open for the postal worker to check.

You have to state on the label (if you use "Yuu Pakku") what is inside, but that's all you need to do. And you just write out clearly in alphabets the recipient's name & address and phone number, and yours too.

I wonder what you read. Japan Post Co.'s official website doesn't say anything about having your contents checked before sending.
http://www.post.japanpost.jp/index_en.html