Justin Bieber's Having a Plain White T-Shirt Sale

Justin Bieber's helping out a new business, and it has nothing to do with music.

The Biebs is dropping a very limited line of white tees with designer/stylist xKarla at a pop-up shop in L.A ... according to sources connected to the deal. We're told Hanes is also involved and the model pushing the threads is none other than JB.

The shirts will sell for $30 a pop and be available for people to buy in person in the L.A. area. The shirts can be purchased in 1 of 3 ways ... order on the Postmates app, show up at a pop-up store that will surface Sunday in the City of Angels or purchasing them online when they become available on xKarla.com tomorrow.

No word yet if JB will also be hawking all that leftover Purpose tour merch.