Hi emsilkina,
Taking water from the tap and using a water conditioner prior to mixing it with the Reef Buffer should do the trick. Although bottled water is likely just as safe once conditioned, there is always the chance that the bottling company adds harmful minerals to the water. If you typically use RO/DI water, mix the Reef Buffer in that instead. While I do not know the size of your tank, a cup of tap water should not cause your reef any damage at all. As for mixing the Reef Buffer into saltwater instead of freshwater, I would assume that this would not be problematic, however, I highly suggest sticking to exactly what the instructions say, which is mixing in freshwater in this case.
Hopefully this helps!