The Playa del Carmen postal service is pretty dismal, particularly if you want to send or receive mail internationally. Strangely, you can always rely on the electricity bill arriving on time!
There is one post office in Playa del Carmen, on Calle 2 near Avenida 20. It is a grimy room which will accept your parcels/ documents, and charge an exorbitant amount for not being able to guarantee their arrival. Also, they do not sell stamps, envelopes, boxes or anything else to wrap a parcel.
Scared at the post office (and no, I didn’t end up sending my package…)
As you can probably tell, this is a bit of a sore point for me! It’s probably my biggest complaint about the country and so I would not advise sending anything of value via the Mexican postal system.
DHL, Fedex and UPS are of course available, and there is a DHL office on 20th Avenue near Calle 10, in front of the ‘other’ ADO bus station.
There is an agent of DHL, Fedex and UPS on 10th Avenue, near Calle 14.
Update: We were very surprised to receive a parcel sent from the US via USPS registered with a tracking number. It took around three weeks, but the tracking was updated on the website and it arrived at the post office safely. We were pleasantly surprised but I still hesitate to recommend sending anything of value.