There are several pharmacies on Cau May Street, but few staff with strong English language skills. If charades fails, try writing down the brand name of the medicine you are after. General items are easy to find, but more specialized medicines may not be available so plan ahead.
There are small post offices in both Lao Cai and Sapa. At: No.6 Thach Son (Sapa), and at: No. 196 Hoang Lien (Lao Cai). There is also a mail box on Xuan Vien if you already have your own stamps. In general, the postal system is quite reliable.
There is free wifi in a number of hotels and restaurants if you have your own device. There are no internet cafes in Sapa, but if you ask really nicely, you may be able to use the computers at Sapa Pathfinder Travel on Xuan Vien. Some of the mid range hotels also have free computers in the lobby.
There are a few ATM’s scattered around Sapa, but remember there are withdrawal limits. Credit cards are accepted in some of the more up-market venues, but many places are cash only. Some of the bigger hotels can exchange cash or traveler’s cheques, but you won’t get the best rates.