Unlike Abba_S, I am a lawyer. I also grew up in the retail business and managed many of the stores owned by the family.
Which cashier gets relieved first is a management decision. It has nothing to do with seniority (in a non-union shop). The front end supervisor gets to decide who gets relieved and in what order.
You write that you worked a 7.5 hour shift and had a 30 minute lunch and 15 minute break.
You did not tell in which state you work. Labor laws vary by state. Here in CT the employer would have to grant you a 30 minute unpaid meal time after 5 hours of work. Other breaks, whether paid or unpaid are not required by law in an 8 hour shift. Breaks are negotiated between employer and employee. In most companies they tend to be standard by classification of employment and may be listed in an employee manual.
As for unpaid work time, that also varies by state. In CT an employee can not be required to clock in or out more than three minutes before the start of paid work or three minutes after work ends. Generally partial hours are divided in 10ths. If you handed in your till in the cash office and clocked out at 7 minutes past the hour, you’d be paid for 10 minutes. If you clocked out 3 minutes after the hour, you’d not be paid for the time. The 3 minute window is to make sure the employee is actually in position and ready to perform labor when the shift starts, not walking to a work station or putting on an apron, name tag, etc.
Other states may have different laws. That said, you are working in an unskilled job and are easily replaced. Call your state labor department and ask about the number of minutes you can work and legally not be paid. If the employer is in violation, find a way to bring it up in a casual conversation, such as you read on the intgernet that a company in X state was fined because its employees were working more than 5 minutes after their shift without being paid. You’d hate for this company to get caught doing the same thing and suffer penalties, when it could be corrected.