If I were you,when you you go to the 'interview' I would raise the request for £50 and ask what it is for and if it is voluntary. I would also point out that you would have found it useful if that information was included in the letter and that other parents like yourself mig be more willing to contribute if the purpose is fully explained.
If you prefer not to raise it in the chat, you could email a query about it,stating you understood such donations had to be voluntary but there was no mention of that. Again, say that you think new parents would find it useful to know what it is for.
Are you sure that nowhere in the letter does it mention the contribution is voluntary?
It may be that in some other documentation or an obscure corner of their website this is mentioned and it is explained what it is for, but I think schools sometimes need to be told explicitly what new parents want to know - they may well adjust next years letter in relation to it.
I like the way a previous poster said their school handled it to boost contributions - by mentioning it at the chat, explaining its use but emphasising it was voluntary.
The poor communication aside, I think that if a family can afford £50 they should give it - yes it is less than £1 per week, less than a cup of coffee. Yes we all wish schools didn't need to ask for the money and had plenty, but the reality is they are hard up and if most parents give what is a tiny sum over a whole year (no need for all in one go if that is hard, and no need at all if it isn't possible) that money can make a real difference and make the experience of school less.......austere.
I think it is disappointing when people who could easily pay less than £1 a week choose not to, either because they think they are making a moral/political point about the fact schools should have more money, or often because they simply know they can get away without paying and prefer to spend that £50 on a few coffees or other little luxuries, which ultimately they value more highly than the children getting a bit broader an education - or expect others to pay and fund that for their child.
Sorry, but off the point.
I agree the communication sounds poor and needs pointing out to the school. And I'm glad that communication aside the OP is willing to give a donation if she can afford it.