Curate, connect, and discover
The real reason why Atsushi shops at the same grocery store as Akutagawa is because the grocery store they both shop at is one of the Port Mafia’s money laundering schemes.
It’s not like Atsushi is completely unaware that it was a money laundering scheme, he just didn’t piece together that it was a Port Mafia money laundering scheme. He shops there anyway because Mori doesn’t know the average cost for most household products so everything is ridiculously cheap and also decent quality.