Punchpass will help you to keep track of unpaid attendances.
The best way to handle unpaid drop-ins is to mark a person as attending class even though they do not have an active pass. You'll see the red flag as a warning, but the attendance will be accepted.
When they buy a pass next time, Punchpass will alert you that they had a previous attendance with no pass.
Just click the box(es) to apply the new pass to the prior attendance(s) when that happens. Here's more about how to manage no-pass attendances.
You always have the option of forgiving a past unpaid attendance if you decide to do that later.