How can we improve WHMCS?

I'm wondering if it's possible to save or recover from the system all invoices that we have deleted as we would need them for accounting purposes since they follow invoice numbering.
I believe if there is a option to delete an invoice that there should be a "folder" where they will be saved as well, similar with cancelled invoices, paid ones, drafts etc. in the folder "Billing-Invoices-..."
Now we have an issue where an invoice has been deleted, it can't be found in the whmcs system any more/on the clients portal, but when a new invoice is created, for same or other client just after, the new invoice doesn't get a number of the deleted one but next one in line. And then we end up with missing invoice in the invoices lineup...
Hope you understand what I'm trying to explain.