The Main post office in Amsterdam is located at Singel 250, Old Centre: New Side, Amsterdam. This is actually on the corner of the Singel near the Dam. In addition to the usual services at a post office, the facilities also include phones, self-service faxes, directories, maps, stamp machines and counters where you can buy packaging for parcels. There is also a counter for collectors' stamps and stationery. The post office closes at 4.30 pm Mondays through Fridays.
In addition to the many services already mentioned, phone cards can also be bought in various denominations from the post office and are typically good for long distance calls. With cash phones you should insert money before dialing but these phone cards from the post office make dialing a little bit easier. The faxes are also fairly convenient there as they have been simplified for easy customer use. Directories and maps can be found both as public reference and as private items that can be purchased for a small fee. The stamp machines are easily accessible near the main entrance and the packaging that is purchased there comes in many different shapes and sizes to suit your individual needs. The Main post office in Amsterdam certainly has everything you will need!