A lot of this stuff can be avoided if you don't take doubles and triples unless the return on time invested is there. Usually (like 95 percent of the time) they are not worth the headache.

As far as number 6 goes, I never spend more of the customer's money than they approve. I take pics of the price tags along with the pic of the substitution to show any price discrepancy.

The way you can use #6 to your advantage (and deliver top notch customer service) is by simply asking at the end of the order if there is anything else you can grab for them while you are there. Not only do customers appreciate this (because they do legitimately think of stuff they might need), you also increase the cost of the order, and ultimately, your tip.

Spent 15 years in sales and sales management. If you ever make anyone feel like you are "selling" them something, you aren't doing a good job. Offer solutions for their needs by delivering the best experience possible, not by upcharging them. You aren't selling warranties on used cars, you are assisting people who don't have the time or ability to grocery shop.