We have just started using it this past week. So far so good.

Only thing I've found is that the web site seems to be a bit unstable. ie freezes sometimes. Or crashes.

Other than that, definitely cheaper. And convenient. We can mail our satchels into the local red box now instead of going to the PO.

Like you, the majority of our stuff goes out by large letter. Just the occasional larger order needs a 500gm satchel.

So gets a thumbs up from us.