Love the inline PDF preview, and overall it's quite clean.
But this is very US centric. But then again, one of my previous jobs existed only because the European subsidiary of my employer did not trust the 50 person strong US billing team to handle European invoicing in a legal way (so we built and operated our own billing system...) - billing/invoicing is tricky to internationalise right.
If you want to consider making it more generic:
Apart from allowing specifying replacement translated text for fields (for many countries this alone makes it impossible to legally use in its current form), a minimum would be changing decimal point/comma (many countries reverse use of comma and decimal point, for example) and grouping.
Also, this doesn't look like it can generate legal invoices for most European countries even disregarding currency, as most European countries have various text that is legally required for a document to be considered an invoice. For starters, the senders name, registered address and registration number (if a company) is required in most instances, and a VAT registration number (if VAT registered).
You can solve that most easily by adding fields for senders/issuers address details to put on the top of the invoice, and a free form text field as a footer, as this varies country by country.
If the invoice includes VAT on line items, there are also usually a legal requirement to sum up the VAT total for the invoice, which would be separate from any other taxes or duties, so you might also want to consider breaking out the different types of tax in your totals. (never mind that there are also rounding issues where specific ways to round are often legally required; in general you ought to be ok for most countries if you add together the unrounded line item tax values and then round, rather than summing up the rounded values - I haven't looked to see what you currently do).
German, Italian and French developers have offered to help translate the site and we're working on redoing how line item taxes are grouped. You can track these issues on GitHub:
There's definitely still work remaining but the more eyes we get looking at it the better we can make it. Please join our Facebook page and keep talking to us.
I'm actually curious at which difficulties you ran into when creating the app without authentication (have been thinking about this myself, just didn't create it yet)
@matthewmacleod That's a good suggestion, we'll work on it.
@lnanek2 Yes, but we're free :)
@ville, Our first concern is to perfect the application, making money will come later.
@peteacc. I’ll need to look into it, thanks for catching it.
@photoGrant If you set the payment terms for the client it will automatically set the due date for you.