Keepa tracks Amazon price history. It works well for Amazon.com. For Amazon.co.jp it has gaps — delayed updates, missing products, no Rakuten/Yahoo!/Mercari support at all.

If you shop regularly on Japanese e-commerce, you're working across four very different sites with no unified price history. I wanted to know: was this ¥9,800 laptop stand expensive last month, or is it always ¥9,800? Is this Mercari listing a fair price compared to last week?

I built PricePulse JP to answer that. The interesting engineering problem was getting consistent price data out of four sites that don't look anything alike.

The four-site parsing problem

The naive approach to price extraction is: find the DOM element that shows the price, read textContent . That works until Amazon quietly renames its CSS classes (which it does, routinely), or you're on a Rakuten shop that a merchant themed themselves.

Each site needed a different strategy.

Amazon.co.jp mutates its DOM frequently enough that a single selector is a liability.…