I have. You just fill up a USPS flat rate box with brass and send it with your order. They will only accept brass in the calibers they sell, so check their website before you fill up your box. The brass also has to be in decent condition (not crushed, etc.). You can mix calibers as long as they sell it. Typically my box will have .223, .308, 9mm and .45 brass in it with the most being 9 mm. There is a video on their site that tells you exactly how to do it.
Man it takes a crap load of brass to fill one of the large flat rate boxes, and it is heavy when you get it full. I think the last one I sent weighed 52 lbs. I try to collect all my spent brass, and any other brass that guys leave at the range, and it still takes a long time to fill a box. I don't know if it is worth the work to collect it or not, but it does give you some exercise picking up cases.