What significant discovery did the detective make regarding the purchases of shoes?

The significant discovery that the detective made regarding the purchases of shoes was that they were all bought using the same credit card. This information is crucial because it can help establish a connection between the transactions and a specific individual. When multiple items are purchased with the same credit card, it indicates a pattern of behavior that may be relevant to an investigation, potentially linking the cardholder to a crime or a suspect's activities.

By identifying the credit card used in these transactions, the detective can gather more information about the person making the purchases, such as their identity, location, and any prior criminal history. This could ultimately lead to further evidence or even a breakthrough in the case being investigated. The other options, while they may provide additional context or detail about the purchases, do not offer the same level of significance in connecting them to a specific individual as the use of the same credit card does.