Thank you everyone for your feedback and suggestions.

The data will be used for nothing except displaying your invoice. Also, i made the urls look dirty with those random characters instead of letting people choose shorturls to keep invoice details private. There's no/will be no page anytime that would list the invoices. So they are all safe.

P.S: pdf and mailing option coming in the next few hours.

Another tack is a "don't take it off the page" javascript thing that generates the invoice without ever submitting the address to the server.