Send reminder and Send receipt are row actions on an invoice — one nudges a customer to pay, the other resends proof of payment. Both work on a single invoice or in bulk.
Send a reminder
Go to Payments > Invoices.
Open the row menu for the invoice and select Send reminder — or select several invoices and choose it from the bulk actions bar.
Send reminder is disabled for voided or paid invoices — there's nothing left to collect. Past-due invoices also get an automatic reminder without you doing anything.
Send a receipt
Go to Payments > Invoices.
Open the row menu for the invoice and select Send receipt — or select several invoices and choose it from the bulk actions bar.
Send receipt only works on invoices that have already been paid.