We invoice with Paypal for our rental as the normal course of affairs. There's even a plugin for Quickbooks that lets you send Paypal invoices straight from your accounting software with no need to actually even visit the Paypal site.
I looked all over for a provider that could provide a similar service at a similar price and it was damn hard to find. Intuit came back with a solution that required us to buy no fewer than three Intuit products.
Off topic...
I am a bit surprised about the plumber and electrician though because in my experience, small contractors in the building trades tend to avoid internet solutions.
Six years ago, we couldn't find anyone in the building trades in our area (rural California) that wanted to even use email - we had to buy a fax machine just to send and receive all that junk (estimates, invoices, etc). When we had some work done last year, everyone was emailing us PDFs and didn't want to deal with faxes if possible.
Of course, that means there's great opportunity for the ones that take just a bit of time - a few years back a simple (crappy actually) website I built for a friend ended up holding three of the top ten places for San Francisco Window Washing (until my friend's genius partner redid it 100% Flash).
