"You make life so fun and easy and I just love you so much. You were so worth the wait and you sure do make me (and Marti) HAPPY," she added.
In the video, multiple clips of James dancing and laughing have been complied together, with Becky sharing an inside look into their relationship for the first time.
Becky's sister Elly, who also starred in The Bachelorette, also took a moment to gush about the couple, writing: "Hahahaha I love this. Happy birthday @bevitron my fave bro-in-law."
WATCH BELOW: Becky & Elly Miles take on the Who Couples Challenge
In comes after the pair went public with their romance on Instagram in February this year.
"Yesterday I took this pic and thought it was hella cute... This is what a happy Becky looks like and it feels pretty damn good," Becky wrote.
"I feel understood, appreciated and loved by you and that’s a pretty amazing place to be... Here’s to many more kombi #vanlife adventures with you."
Both Becky and Elly starred alongside each other in last year's season of The Bachelorette, and while they didn't end up with their respective winners, Pete Mann and Frazer Neate, the sisters have since moved on and have put everything behind them.
Speaking on The Real House Husbands Podcast hosted by The Amazing Race's Tim and Rod Sattler-Jones, Elly recently weighed in on her and her ex's relationship.
"I had the highest hopes, I was very naive and I guess caught up in hoping the best for the situation."
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