USPS moneysaver (has anyone used it?)

[kendricks]

This thread is a bit off-topic, but I couldn't find another section for it here.
I've been using https://labels.vc/ for a month now, I buy shipping labels (usps priority) from them all the time to send some detail parts through usps inside usa, saving a lot. I have 2 questions:

1) has anyone of you here used sites like this?
2) are these sites legal? customer service assures me that they have official contracts with usps.