How to confirm a payment was made?
I'm not sure about Nbitcoin node's API, but I could help you on a blockchain explorer API.

I would recommend Blockr.io, they are fast and trustable. Use the following code to check for confirmed transactions: (output is in json)

http://btc.blockr.io/api/v1/address/balance/198aMn6ZYAczwrE5NvNTUMyJ5qkfy4g3Hi,1L8meqhMTRpxasdGt8DHSJfscxgHHzvPgk?confirmations=1


Change the confirmations tag to whatever amount you want. I highly advise against 0.