Not sure if you'll see this, but we just implemented line items on invoices now. If you want to try it out, drop us an email at contact@billerly.com mentioning this conversation and I'll make sure your account has enough invoice limits to try it out!
Feedback on sample invoice: tax should be a line item and include a description of the tax. Adding it below 'total due' is confusing. Not having a description makes it even more so.
I use PayPal for invoicing (free, you don't have to receive payment through them) with Google Calendar reminders.