So my choice is to harass/insult a minimum wage employee and distract them from their already-overloaded and already-minmaxed job so that I can spend more, get in my vehicle and spew carbon into the atmosphere to......give money to a family of ultra-billionaires instead of one ultrabillionaire? And that's helping my local economy?

Why are you harassing and insulting them in the first place? Nobody taught you how to speak politely?

The GP said to tell the employee to get off his butt, which is usually interpreted as rude.

They didn't mean you should literally say those exact words. You could say something like "Would you mind checking it for me yourself? I'd really appreciate it."

Not sure I’d consider Walmart the kind of local business being discussed here...

Walmart is not a local shop. They have a pretty large software dept i house and you could probably check availability online yourself.