Would it be possible to have customers come more frequently? I understand that currently you only have a single customer spawned in at a time as it is now, but it would be a much more natural way to increase player challenge by having more customers, compared to decreasing the customers' patience. Having the customers queue up could be difficult to program though, but this would give them a reason to have less patience. The rate at which customers come on a given day could also be determined by how many customers were satisfied the previous day.
Maybe once you can upgrade your store, you could have room for multiple customers at once, which would increase the potential for players to earn money even further while making it an added challenge to satisfy all customers.