I did this a lot. It's not particularly cheap, but it's convenient. The interface is a bit slow too.
A couple of tips:
Never try to compose a letter using their WYSIWYG thingee... it's a pain. Do it in Word or something and upload.
I found the electronic return receipts, which are cheaper, not as reliable. Twice I just never got one, even though it was reported as delevered, and once the RR was unreadable. The mail carrier has to scan the signature using a little machine, and some of them do it correctly and some of them don't. So if you don't mind the cost, the green card is more reliable. That said, most of my other ones worked, and they refunded me for the ones that didn't.
I tend to be someone who does better getting things done immediately rather than waiting for the next time I go out to remember to take things to the post office, so in the end it was worth it to me.
