The Home Depot complaint: charged full price for culled lumber

I went to purchase 3 each 1 x 8 x 6′ ship lap boards marked with purple paint which indicates it is marked down because it has defects. I brought it to the attention of the cashier but he failed to mark it down. I didn’t notice it until I got home. I called and they said I would have to drive back to the store (30 minutes away) to get a credit. I asked to speak to a supervisor. They apolagized and said the same that I would have to come back for a refund. I explained that I came in at 6 ocklock in the morning so I could get to start on my project. Mike said I apolagize but you have to come to the store. I replied so I have to take an hour and a half of my time and spend 10 bucks on gas to get my 15 dollars back that you overcharged me. He said yes. It wouldnt have bothered me so much if this was the first time but this has happened to me several times in the past few months