According to the official post office website there are 63 post offices in Kuala Lumpur.
In addition to sending mail and parcels, you can also pay your bills at post offices but check if they accept credit cards. The one at Plaza Mont Kiara only accepts cash.
A quick glance at where some of the post offices are:
Ampang Park Shopping Centre, Berjaya Times Square, Midvallley Mega Mall, Plaza Mont Kiara (old buildings), Suria KLCC.
Most Post Offices are open Monday to Saturday but opening hours vary greatly. Some are also open on Sundays such as the one in KLCC and Midvalley. The Link below gives you specific details and a phone number to call to double check before you go.
For more locations and exact addresses and opening hours please have a look here: