An invoice is automatically generated for each despatch. Note that the despatch note and invoice numbers are the same:

The print an invoice option creates a PDF of a given invoice to print or email to your customer. The system warns you if the invoice you select has already been printed:

The print many invoices option will create a PDF containing many invoices.  If you select 0 to 99999999, and only print unprinted then Herschel will find the new ones for you:

The print summary invoices option allows you to combine many invoices into summary invoices.  It creates a PDF of the invoices to print or email to your customers.  It can also generate summary despatch notes: