I basically use Freshbooks.com as an invoice generator. This is way cleaner.
Some polish-level things that would make it cleaner to use for a SaaS product:
* There aren't really "quantities" or "rates". You are a customer of my product, which costs $X amount, so there is just a line with cost $X.
* Needs to comma-delimit amounts greater than $1,000
* Want to be able to put my logo where "INVOICE" is.
* Want to be able to include an "Amount Paid" below the "Total", and then have "Amount Due" under that, which is Total - Amount Paid.
Also, I want to be able to save invoices (local storage or server is fine), generate a new one this month from last month's per customer, etc. etc.
Feel free to email me (harry@periscope.io) if you want more feedback or some examples.
Also: Charge your customers money!