My basic rule of thumb is:
Will the client make money from it, or save time from it?
Then charge for it.
But l’d also say you have to balance pricing as a freelancer. If it’s something that takes you little extra time, but adds value for your clients, consider throwing it in.
But as a bonus!
Make sure they know what it would normally cost.
I see a lot of freelancers going the extra mile, And no-one even knows how big a deal it is.
It’s likely this “little thing” actually took you years to learn.
But if you want to offer a high ticket service and charge more than your competition you have to get used to giving away the kind of value for free that others are charging for…