Flat-rate YCH's sell awesome and are a simple and accessible way for customer and producer to get a good result. The artist can just work out some old sketches or designs they haven't been able to figure anything to do with yet and the consumer gets easy and rather cheap artwork that is often a great character representation because they're usually a tad simpler than usual commissions.
Bidding would be great for you and several other artists with a decent following in certain situations (holidays, sales) because the stir gets really big and people get truly excited for your work. Often the people who lose will be more considerate of buying from you later as well in this case because Gambler's Fallacies and overall excitement.
I personally adore standard commissions, though, because the artist gets to spread their wings and experiment and has the chance to succeed and fail and that's just really exciting for me.
Sadly I'd had little luck bidding except with teh sketchbook commissions I did. All the YCH I did before went for tarting bid was like, a full color for 50$. Ouch. So I'm a bit wary to bid, mostly I posted this to see what my watchers were most interested in.
While I like the standard commissions, these YCH are great for poses I've WANTED to use for a while, but had no reason to and gives me even greater reason to KEEP sketching!
I hope Ahk doesn't mind but YCH's are where the artist makes a basic sketch and offers to fill your character in in that space for a fee. A standard commission is started from scratch. Essentially, you buy the pose/surroundings/context and your character is placed in it in a YCH.
The most recent non-human animals that I have been interested in are rats and Patagonian maras. I wouldn't recommend making YCHs of the latter, because I'm pretty sure I'm about the only one that is interested in them!