Add to Buyee

• Can I shop at Amazon Japan or other shops for "Add to Buyee" through Buyee without using "Add to Buyee"?
• Is there any item cannot be purchased through Buyee on Amazon.jp?
• What is the difference between stores for "Add to Buyee" and other online stores within Buyee?
• "Add to Buyee" could not be operated normally.
• What is "Add to Buyee"?
• I have installed "Add to Buyee", but it does not appear when I visit stores for "Add to Buyee".
• I can't install "Add to Buyee" (the add-on).
• Is it possible purchase Pre-order items by "Add to Buyee"?
• Is it possible to add multiple quantities of the same items into shopping cart when using "Add to Buyee"?
• Can I purchase the pre-order items and other items at the same time?