The point is that flying with blocked sinuses means that you probably also have blocked Eustachian tubes as well. The reason for not flying is that you may well cause barotrauma, particularly to the middle ear which will put you out of action for a lot longer. (I knoe I did it in the UAS when young and foolish!)
If you have to use a temporary solution try Karvol or menthol and eucalyptus inhalations, broken into a tea towel and inhaled over a bowl of hot water.
These preparations have no adverse side effects and are not proscribed under any AIC I can find.
If you have chronic sinus obstruction you need to get referred via your GP to an ENT surgeon.