The old guy sounds like he deserved it lol! There's no excuse for someone to call you a "C" word even by a customer that is upset, that is abuse towards staff and there should be a zero tolerance policy towards this kind of unacceptable behaviour even if he was an old guy, but it obviously would have been much better if you'd handled it better, in that situation it would have probably been best to warn the customer to calm down and not use abusive language or leave the premises and if he continued he should have then been asked to leave (any good manager should have supported you if you'd done that while keeping your cool, but it doesn't mean there was a good manager there). It's difficult for us aspies to handle situations like that however when they're actually happening and I had a similar situation when I worked in a pub years ago that I didn't handle well (a pub is similar to a bar in the UK for people from the USA).
It's a difficult one deciding whether to mention it. It's good that you worked as a volunteer for a charity shop and it's a shame not to mention it, the only time it would be an issue is if you tried to get a reference so you'd have to avoid doing that and if you were asked why you left you could tell a "white" lie, but if you were only there for a very short time it probably isn't worth mentioning under the circumstances. I would definitely mention working as admin for charities however as it shows what kind of person you are in a really positive way, it shows that you are determined to fill your time and work even if it's without pay, this shows great willingness to work and you are therefore more likely to be someone that puts effort into your work. It's also okay to mention Oxfam and if you're asked why you left you can even tell the truth as this is a reasonable reason for leaving (I think most people would have done the same).
