Note that the 10% discount only works for some items as mentioned. I recently bought a few parts from 4-into-1 and wasn't granted the 10% off on the TRR400 fork caps so I messaged Rufus. His reply:
"The 10% off coupon works for about 90% of the items we carry. The only things it doesn’t work on are brands that are price controlled like a Biltwell, Motion Pro, TTR400, etc."