Walmart's $3.3 billion acquisition of Jet.com is now official. Next comes the hard part.

Walmart's $3.3 billion acquisition of Jet.com is now official. Next comes the hard part. ,

Jet.com headquarters in Hoboken, N.J.

Done. Deal.

Walmart announced on Monday that its monster $3.3 billion acquisition of e-commerce startup Jet.com is official. Walmart CEO Doug McMillon broke the news with a photo posted to Instagram on Monday. Startup-y!

I've been waiting a month to put on this shirt. Today is the day. Happy to officially welcome Jet.com to the Walmart family!

A photo posted by Doug McMillon (@dougmcmillon) on Sep 19, 2016 at 1:01pm PDT

In a blog post, McMillon explained the deal by saying, “Walmart and Jet.com obsess about saving customers time and money. By joining forces, we’ll be better at doing both, creating seamless shopping from app to site to store.”

After the honeymoon ends, the challenges for both sides will be many.

For one, Amazon continues to gain market share in the U.S. and doesn’t appear to be losing any steam, while Walmart’s e-commerce growth has decelerated for most of the past year and a half.

Jet.com also has to prove that it can develop a loyal customer base over many years, not just one powered by heavy advertising spending. And for Jet CEO Marc Lore, who will also run Walmart.com, there’s the tough task of bridging the gap between the company’s e-commerce efforts based in Silicon Valley and the core brick-and-mortar operation, which still accounts for the vast majority of revenue, in Arkansas.

There’s a lot of money riding on both sides overcoming these challenges.