"Hello, welcome in" as opposed to "Welcome"

Solution 1:

Greeting someone by saying “welcome in,” is hardly a novelty:

I passed by the crowded gate, and though the keeper was preventing the entrance of the crowd, finding that I was an American stranger, the porter said, “Welcome, welcome in,” and opened the gate. — Asenath Nicholson, Ireland's Welcome to the Stranger, 1847.

“Young master elf, welcome in, welcome in all.” He opened the door and ushered them all in, shutting the cold out quickly behind them once they'd entered. — Brad Higgens, Talisman of Blood: Book 1 – Shadow of the Gryphon, 2012, 175.

Welcome in out of the rain! Today the shop is open from 13.00-16.00 and I, Andrea, will be in place to offer corset-related expertise! — PicBon

Welcome in out of the cold once more my friends. Blow a nice warming breath onto your blue tinged fingers, and pull up a stool around the fire. — Tim the Tum blog, 31 Jan. 2012.

What does seem novel is that you’re hearing it as you enter a shop, especially if it’s in a mall, where you’re moving from one climate-controlled space to another. Traditionally, the greeting is given as someone holds a door for you, especially in inclement weather. The in is directional, i.e., accenting that you’re moving past an obstacle or coming in from rain, cold, etc. If none of these apply, then I would imagine the implied barrier guards what the store employees hope is a pleasant, relaxing, or luxurious atmosphere, at least better than whatever is “out there.”