Set a per minute rate for custom videos. The usual average I’ve seen most models charge is $10/minute, but you can charge whatever you’re the most comfortable with, whether it be more or less.
I personally only take custom video orders through ManyVids. It’s an escrow system, where the site holds the payment until you send the custom, and honestly it’s just a lot more hassle free and safe for everyone involved. There is a 20% site cut through MV, so you only get 80% of what the member spent, but in my opinion it’s definitely worth it. I raised my prices to cover the site cost so I can make what I want with customs and haven’t had any complaints. Since you already have a C4S store, you should definitely open a ManyVids one if you haven’t already just to diversify your income since you can post the same videos from your C4S. Your other options for payment is either iWantClips customs or IndieBill, unless you go with offsite methods, which I really don’t recommend doing, especially when it comes to custom videos. Also, don’t use C4S’s tributes to have members pay for custom videos either.
After it has been sent to the member who bought it, unless they paid an extra fee for it to be exclusive, you can then put it up in your store at normal price. I know some models wait a little while after sending it to post it in their store, but I do it all at the same time since I always send it through MV and I’ve never had anyone complain. Most guys ordering customs are familiar with how it goes, and I’ve even had multiple guys tell me they hope their custom sells well for me.
Edit: I also wanted to add, on top of the base rate, you can, and should, charge more for certain acts that you may not normally do and anything that makes filming more complex. If you accept a lot of customs, selling them in your store can be a hit or miss, so you want to make sure you make however much is worth it to you before even making any extra sales. I’m super picky with customs nowadays and only film what I’m about 98% sure will resell.
Also, if they want any props or outfits that you will have to purchase, add that cost onto the total price for the custom.