The donation is generally stated as in lieu of flowers, so the cost of a decent bouquet at a nice florist is a good guide.
In my view, if you haven’t been part of the decision-making around the funeral (and hence party to the ‘family flowers only’ choice), then the notice is aimed at you and so you do not send a wreath. I’m guessing that is what intended by the previous poster who said if you have to ask, then you don’t send.
In my country, the undertaker informs the charity that donations are requested in memory of X. And you could make a donation online, with a message that is passed to the next of kin. But if theres a collection option at the funeral, that is simpler.
If the deceased was religious, check if there is a sympathy card tradition in that religion. For Catholics, it’s very common to bring a Mass card to a funeral. Otherwise I’d probably write a sympathy card or blank card and ask your daughter to leave it with the MIL, so you can be represented at the funeral.
In general though, it’s a bereavement, it’s horrid for everyone. Etiquette can be tough to figure out but as long as you act with kindness and good intentions and compassion, you probably can’t go far wrong.