Rakuten shares were trading up 18% at 1,471 yen at 02:28 GMT - the biggest rise in 17 years - after earlier hitting a daily limit high of 1,545 yen. That brings Rakuten's share gain from Friday to almost 30% and lifts its market cap to 2.1 trillion yen ($19.4 billion).
The deal is a funding injection for Rakuten's investments in logistics, where it is competing with Amazon and SoftBank's newly bulked-up internet business, and in mobile, where it is taking on the three incumbent carriers.
"Rakuten could receive significant help from the expert in logistics services," Jefferies analyst Hiroko Sato wrote in a note, referring to Japan Post, which operates 24,000 post offices nationwide.
The mobile business "has been a huge drag on its valuations and earnings for a while now but one which will see costs peak this term," wrote Asymmetric Advisors analyst Amir Anvarzadeh in a note.
Investors also warmed to potential partnerships with backers Tencent, the world's largest gaming company, and Walmart, the world's largest retailer, which is also a buyer in Rakuten's share sale and is leading a digital fightback against Amazon in the United States.
(Reporting by Sam Nussey and Junko Fujita; Editing by Tom Hogue and Muralikumar Anantharaman)