Sorry not sure how to edit the wiki but here are some responses
Q: Are any discounts available?
No, as Oyster and Contactless is seen as a discount from buying paper tickets - which do cost more. However with fare capping if you go Pay-As-you-Go the maximum you will pay is the cost of a travel card unless you get a bus first as for some reason that does not seem to count, not sure if this has been fixed recently.
Q: Can I get a receipt?
If you buy from a counter or machine you can request a receipt.
Additionally
If you are buying an over ground ticket but also need to use the tube it is often cheaper to get one that includes a travelcard for example from Slough to London return is about £17 however with travelcard it only adds up to £21 which is cheaper than using contactless.
Also if you are getting overground tickets and know what you are doing you can get something called boundary zone tickets which may be cheaper. This is because you buy travel cards for a zone not a station so it is in effect an imaginary point on the track. A situation where this maybe useful is if you have a multi-day travel card but decide to go out to the countryside you can use the travelcard to get to the edge of the boundary zone and then only pay from there to your destination. you can only get these from ticket offices the person there will usually know what to do.