1. It would be better if the application could work without an internet connection. What's stopping you from caching the stamp data and then syncing when the user has an internet connection? This would make the product more attractive to both merchants and users. Merchants wont have to provide free wifi to ensure the stamping process is fast and smooth, and users will not have to bother with connecting to a wifi network. The technologies you are competing with (QR codes and NFC) can both work without internet connections.
2. Security. I did not see this mentioned on the FAQ and this is probably something that merchants will want to know. For example, is your system immune to "replay" attacks? If a user managed to record the capacitive stamp, what's stopping them from re-sending it to the servers to get more stamps?
3. Customers may not always want to share their email address directly with a merchant. Why not provide merchants a way to send deals to customers through an API i.e allow them to send deals to customers without exposing the customer's email address?