For larger purchases like expensive carpets, or Bhutanese traditional dress, some legal artefacts, thangkhas etc. you can use credit cards. For trip of 14 to 15 nights, I would say bring at least 400$ in cash. You may or may not use it but it is good to have. Debit/Credit cards works here but only bigger establishment accept them but not the smaller vendors.
As for ATM or Debit cards, ATM machine are not reliable in terms of its workability. So bring some cash for your daily expenses such as for beer & hard drinks. For easier transaction, you may change into local currency in airport or bank.
Ensure to have exchange receipt, if you have surplus at the end you may change it back.