counter for packaged goods (esp. powdered medicine)
Used to count small paper-wrapped packages, especially powdered medicine in traditional Japanese pharmacies. Not a general counter for all packages.
Please take three packets of this medicine a day.
I received five packets of cold medicine at the pharmacy.
包 counts individual wrapped packets of medicine, while 服 counts doses (one administration, which may contain one or more packets).
包 (ほう) is a narrow counter used only for small wrapped medicine packets, whereas 包み (つつみ) is a broader counter for any wrapped object, such as a gift.
From the kanji 包 (wrap, package), used as a counter for wrapped items. The reading ほう is the on'yomi.