Well six feet is kind of a joke anyway because the virus spreads much further distances, but setting aside that, when you get to checkout, how do you pay and still remain 6 feet away from the cashier? The day before I got sick, I was at a Sam's club stocking up and I was pretty surprised that I still had to physically hand my membership card to the cashier, they still had someone helping me unload my cart onto the conveyor belt thingie standing a foot away from me and reaching into the cart the same time, they weren't sanitizing the credit card keypad between customers, and I still had to give my receipt to the guy who checks it before you leave the store. So even if you "social distanced" in the aisles (which wasn't really possible they way they had it set up, although your store's idea of one way lanes is a good one), you still got exposed to three staff members at the end plus every customer that had touched the "okay" button on the credit card machine. Admittedly that was a while ago, since I've been sick for several weeks, and maybe they've gotten better over time, but at that time the guidelines weren't any different than they are today.
Where I got the 30 seconds is that the CDC is treating exposure time as a few minutes, which seemed high to me, so I revised it down to 30 seconds. Per the CDC website:
"Data are limited for definitions of close contact. Factors for consideration include the duration of exposure (e.g., longer exposure time likely increases exposure risk), clinical symptoms of the patient (e.g., coughing likely increases exposure risk) and whether the patient was wearing a cloth face covering or facemask (which helps block respiratory secretions from contaminating others and the environment), PPE used by personnel, and whether aerosol generating procedures were performed.
Data are insufficient to precisely define the duration of time that constitutes a prolonged exposure. However, until more is known about transmission risks, it is reasonable to consider an exposure greater than a few minutes as a prolonged exposure. Brief interactions are less likely to result in transmission; however, clinical symptoms of the patient and type of interaction (e.g., did the patient cough directly into the face of the HCP) remain important. Recommendations will be updated as more information becomes available."