Got back from this border run today. I didn't use the private bus service and instead just took Green Bus. Information for anyone who is interested:
They give you this little business card when handing back your passport after getting exit stamp at Thai immigration. Transfer bus across the bridge is 25 baht. After 4 PM there is a $1 overtime service fee to get stamped in at Laos immigration. Shared songthaew charges 80 baht to get to town. Stayed two nights in Huay Xai. At return to Thai immigration there was girl a few people ahead of me on her second land border run (I don't know how long total she'd been in country) who got sent to interrogation room. She wasn't dressed very well at all and was a black Westerner. Two Europeans who were dressed in typical backpacker attire breezed thru the other line so I switched lines. IO asked me where I was staying (I said Chiang Mai) and stamped me thru. I was dressed nicely, did have my TM.30, an "onwardticket" bought from one of those sites, 10k baht cash, and an itinerary, but wasn't asked to show anything. The Songthaew (60 baht) took 10 min to fill up and in that time the black Western girl was eventually let through though she was using van service and the guy from the company had gone in to assist with her. Apparently they had asked to see onward ticket and she didn't have one.