How to allow multiple submissions?

How do you let someone submit the another from.  If they bought one thing and then two days later wanted to do it again. 

Are your users unable to submit the form multiple times? If so, the Unique Submission or Unique Question feature is enabled in your form. 

How to Set Unique Submissions on a Form

How to set Form Limits Based on a Unique Question

You need to disable them to allow multiple submissions.