As a general rule, an invoice will be generated by ZumeForms when a form is submitted that includes a payment through Stripe. The invoice is generated once when the form is initially submitted.
If you want to be able to re-generate the invoice after the first submission, then you will need to include some script on the form. This can be triggered by a form event, or by a button on the form.
The script is:
alert( ‘generate invoice = ‘ + $(‘#zf_generateinvoice’).val());