There are several Farmacias in Playa that should be able to help you get a decongestant that will work. While the government made PSE illegal (which may have led to dangerously more potent crystal meth) Mexican Farmacias have many drugs available OTC which require a prescription in US. As an earlier post mentioned you don't want to get a product often confusingly labeled as Sudafed, but containing phenylephrine (PE) which never worked for me.