Hello and welcome.

Would you try rubbing a little smidgen of talcum powder over the signature with your thumb, and take a photograph of it at an angle, so that it can be seen?
Some of us might recognise it.
I don't recognise the work at all, I'm afraid.
Values are very subjective, we don't tend to discuss them, but for a charity shop, we might be more helpful at suggesting a ball-park figure, but as I'm sure you know, it would then be up to your customers to decide to pay that amount. What a charity shop can get is not always the top price.
(very frustrating - I was a charity shop volunteer myself, doing the bric-a-brac.)