It would have been an easy answer a couple of years ago. ARA, but they don't deal with the general public anymore. You need a tax ID number now.
Midwest Refineries get mixed reviews. While most seem happy, it seems they have some shady practices at times. If you send them under 3 ozt. and expect 95% payout it will probably be closer to 85% (from what I've read). They don't assay under 3 ozt. and they don't consider anything plumb gold if it isn't assayed. Example: 14k = 12.5k, 10K = 9k ect. Expect 80-85%. Still better than the cash for gold places.