I mean you might have to negotiate a bit but yeah, a simple professional statement like “My rate for custom enhancements is $X/hr” is not going to ruffle any feathers. They might not bat an eye.
The thing is if you agree, now you have to deliver. Be sure it’s something you want to do. If the project is open source because you don’t want to be a business, then be careful about letting a little quick cash change your mind.
I wouldnt worry about the license, unless you licenses yourself into a corner. MIT is great for this.
Secondly, yes. The biggest challenge I have seen is getting on "VENDOR LISTS". Vendor approval is a huge PITA. master agreements, proof of insurance, etc.
You don't have to deliver a result or continued service. You are paid per hour not per feature. If you can only offer an hour support you can only charge one hour. If you have too many clients you can decline new work until you have time. Per hour work is limited for total pay but clients expectations are limited by time.
500/hr more like.