I'd highly recommend paying upfront for an invoicing script (that has the full source code if possible) and then if you're needing modifications done, to pay a programmer a few bucks on say freelancer.com, for example, so that you're getting something that fits all of your needs. blinksale and freshbooks are good services in their own rights, however they don't allow for hardly any true customization. Take a look at http://www.freelancesuite.com/ and http://www.freelancerpanel.com/

Either that, or just hire a programmer to develop something from scratch. You'll save tons of money, long term.