Once you know you've lost the customer and it isn't worth it to earn them back, sincerely wish them well and mean it, invite them to call or visit if they ever have any questions you can help with, and let them know you'd love the opportunity to earn their business in the future. If appropriate, ask for leads in a respectful way. Thank them for their business, show them out, smile, their last impression should be that you're a stand up person and a consummate professional. Then, take no more than 5 seconds to be grateful for the account and the people, then put it behind you and move forward, wasting no further time on the past. Yesterday's work is done and can never be amended. You can't do tomorrow's work yet. That leaves you only to do the rest of today's work well, so do it! That's my current rule until it's proven less advantageous than another way of life (in terms of happiness, golden rule, and profit).
Based on your description of the people, I'm guessing (not naming names here) that you didn't go to that place in union gap. Most likely the strip mall on summitview. They were pretty relaxed and really enjoyed telling me about how much they love getting to neutralize MA spills on their carpet with baking soda. Slight chance for the building with warehouse in the numbered streets. They were the only one I could get to before they closed one day when I needed FAS-DPD chems during a SLAM ... great inventory on reagents, but almost every bottle I checked expired within a couple months.... TFTkits guarantees for a year!
I almost bought a K2006 from the union gap place, but it's expiration date was just under a year, whereas the Amazon seller promised me just a couple months shy of 24 months, and carried through on it.
Back then I didn't understand that TFTkits used all Taylor reagents and had amazing customer service and 12 month chem guarantee. As long as TFTkits continues to be owned by the current owners, and continues to buy fresh chems direct from Taylor - I now view the TF-100 as the best deal in test kits hands down, even if you pay for expedited shipping. I love my TF-100 and my K-2006, but there's a clear winner in value and it's the TF-100.