with coupons and discounts you would have to set the appropriate (simple) discount for each item or multiple items (you can set multiple discounts of course as appropriate). However, depending on what exactly your prices are supposed to be, that won’t be a fast job either.

That said, i suspect you do not actually really want to apply discounts, but really have the item prices reduced to start off with between certain times, in which case the way to go about that is to write some script that filters the custom post type (in this case wppizza) meta data – that’s where prices are stored for an item.

you should consult the wordpress codex regarding filtering post type meta data

Same applies here too though. There is no “simple/fast” shortcut to do this sort of thing