flat price is good and maybe 16 for an adopt fcfs (first come first served) and then maybe add 4 dollars if they want a nsfw edit ^-^

also idk what ota is :P

OTA is offer to adopt <:

maybe a mix of the 2. and for real intricate designs do like 22 and +3 for nsfw

nsfw adopts and customs

Flat prices are good, if you do decide to do that, make sure you make it clear what monetary system your using, because there's a big difference between $15 usd and 15 Peso

Not sure what PWYW is

I can't get enough of auctions, half the art I have came from auctions. You might keep doing them, for your flat priced art that doesn't sell within X amount of time.

Customs like ref sheets? Always good to have the option to do that, but don't let anyone bully you into giving away refs for free. There are other artists out there that are willing to do free ref sheets when they are starting out, you've already got a base style that people can see, so they can't use, "I want to see your ref sheets before I'm willing to pay."

If you do go flat prices, it really depends on how much work, and how long it took you to finish. If it takes you an hour to finish a single piece, you don't want to sell that for Five bucks, that's less than min wage in the states. I'd say generally around 10-15 for simple things, and anything thats experimental, probably around 20-30, assuming it looks good.

OTA as in what? Are you talking they note you how much they are willing to pay for it, and you say yay or nay? If that's the case, I think all of your adopts should have the potential for that. Example, you make a monster girl, she doesn't sell within X time, and you're about to put her up for auction, you could put it up as an OTA before the auction, and see if anyone buys her. If not, send to auction.

And since I see the NSFW option mentioned, I'd say anyone who wants an NSFW version of an adopt or custom, add 5 bucks to the price. IE, if the base price is 10, with nsfw is 15