[Advice for people in the US, primarily]
Another more indirect way to negotiate is via credit cards.
If life has been good to you and you’ve got good credit, it can feel a bit off negotiating on price. Premium credit cards that offer rewards that you are likely to use can help address this. Not only do premium credit cards typically offer the best rewards programs, but they also restrict themselves to people with strong credit scores. This is important because they are making their money from well-off people who are too lazy to pay their bills on time vs. people in a tight spot who may have overextended themselves due to no fault of their own.
Anyway, my partner loves to travel (or did, back before the plague) and she loves to optimize things, so she always has a handy list of deals offered by our current cards as well as other cards offering new member promotions. When I go to buy a synth, I check with her, pick the appropriate card, and pay. The end result is that a combination of synth buying and strategically staying at the right cheap hotels results in vacations that cost us quite a bit less than you’d expect.
Another thing you can do if you have good credit and did miss a payment is call up the CC company and nicely ask “if there is anything you can do about that late fee, I always pay on time.” 99% of the time they thank me for being a good customer and erase the late fee.
I tend not to negotiate with retailers directly because I have too much experience with corporate negotiations which are essentially strategic psychological abuse, and unfortunately it is far too easy for me to slip back into that mode.
Something else to be aware of: retailers who primarily serve wealthy clients often have implicit policies to screw their rich customers as hard as possible while cutting deals with more ordinary folks. It is possible to dress in the most expensive fashion brands while paying essentially Target/Walmart prices. It takes a great deal of skill, strong fashion sense, and a delightful personality to pull this off but it is possible.