From what I've seen, the official stance is to only enter the credit price. The 3rd party price imports should all be credit, so entering cash will just make the manually entered prices not line up properly with the imported prices. The assumption is that people in areas with both cash and credit prices understand that the cash price will be lower than what they see in the client. And from the few places I've been with this setup (it may be different elsewhere), the price difference at a given station between cash and credit is always the same. If it's 10 cents when the credit price is $3.40, then it's also 10 cents when the credit price is $4.00.
Having a separate set of prices for cash might be nice, but it also means a more cluttered display. There is already talk of adding other types of gas to the display. Consider doing that and adding a cash price for each type. I think that may just be too much.
Considering probably 95% or more of gas stations include convenience stores (even if it's not one you can enter, but have to request them to get the item for you), I'm not sure if having a display saying that one is there is necessarily worth it. At least in the US. I don't know how it is in other countries. In the US, gas stations really don't make much of any money on selling gas. Their income is mostly by getting you into the store to buy something there, so not having stuff for sale isn't a good option in most cases.
Omaha419, in many places, debit cards are treated as cash for those lower gas prices.