Our clients use Onsight to issue sales orders not tax invoices. Onsight automatically sends out the sales order to customers. Some clients connect Onsight and their accounting system so that sales orders in Onsight get automatically imported into the accounting system ready to be converted into invoices. Then they send the invoice from the accounting system.
Whilst it is possible to make the Onsight sales order look like an invoice, you need to make sure that whatever legal requirements there are for invoices in your jurisdiction are met.