Saw this on Facebook. From my general research their bikes are excellent value already, they're the same guys (Polygon) that make the good ALDI mountain bikes. Their full range goes much higher than just $399 bikes though.
On a given bike's page they'll often have a comparison to a similarly specced bike from another major brand highlighting how they're around 20-30% cheaper for a similar bike. Their standard prices are usually not reduced much further. Sometimes they'll have long running 5-10% discounts on select models, but I guess this gives you the opportunity to get any model you want with a similar discount.
They have some bikes on eBay too, so might be worthwhile seeing if a the 10% off codes work,
Might be cheaper