There are two reports that are useful for seeing which registrants have partially or unpaid invoices.
1 The A|R Report gives data by Individual or Family Account and provides links to the student (or the primary student if you are looking at a family account) to see the unpaid invoices by individual or by family. This report also gives you a tool (send email link in upper right corner) to send an email message (not the invoice) to the families using the 30 day, 60 day or 90 day filter.
2. The Invoice Report allows you to filter for invoice status = unpaid, and again for status = partial, and export the results to Excel. This report uses multiple filters, allows you to select a filter type from the drop down, click add filter. select a second filter such as date range, enter the dates, and click add filter, then click run report. This report also gives you a tool (link in upper right corner) to send a copy of the invoice itself (not an email message) to the listed invoiced student email addresses.