Disregarding the express checkout option
There’s a reason grocery stores have implemented the “10 items or fewer” overhead. To reduce waiting time at the checkout for customers who’ve only bought a few items, and to improve customer flow in general.
If you blatantly disregard this rule (yes, it’s a rule, not a recommendation), you’re just being rude and forcing others to spend more time inside the store than they would like to, especially amid the coronavirus pandemic. Making them wait in line with one or two items while you unload your months’ worth of groceries is a major faux-pas. They have every right to use the express lane while you have every reason to be reprimanded for your disrespectful behavior.
Taking forever to pay
Sure, sometimes, you might not get a hold of your wallet fast enough to pay for your groceries. Your bag is full of other things, making it difficult to find your wallet, you have insufficient funds etc. The point is that it sometimes happens. But when you’re taking forever to pay, not giving a thinker’s curse that the cashier and a bunch of other people are waiting for you to finish so that they can move on with their life, then that’s plain rude.
No one is saying you should be Flash, but to avoid this sort of situation, it’s best if you keep your payment method handy, especially if you are paying by check. Speaking of which, stay away from these 11 Purchases You Should Never Make with a Credit Card.