I don't know what your requirements are (U.S. versus Foreign, etc.), but WePay has a nice email invoice solution:

https://www.wepay.com/online-invoicing

Interestingly enough, I'm just finishing up an email invoicing system that I'm planning on open sourcing. It integrates the Stripe button and can be hosted on Heroku.

I'll shoot you an email when it's up on github if you're interested.

Hi David, that feature is on our list of things to add soon. If you shoot me an email I'll let you know when we have something for you.

I use dwolla. It charges only $0.25 per transaction.

So for an invoice of $8000.00 I pay only $0.25 in transaction fee.

you don't use any accounting/bookkeeping software? freshbooks, xero, etc?

Zoho invoicing works.