Looking back I think Neal was upset because I think his engagement to Tamara was shortly before getting that postcard, like maybe a week before. I have no evidence, of course, but Tword wise:
–October: August comes back from Pukhet and sees Neal in NYC. Neal “meets” Tamara on the street
–May: Emma breaks the Curse and August sends him a postcard as a wooden man when magic comes to town (all on the same day). I’ll give the pigeon one or two days to fly to NYC from SB but it’s all in the same week at the latest.
–“November or December” (in SB only, based on it being Halloween in the Doctor and how quickly Emma and Snow returned after that and then they all went looking for Nealfire): Emma, Henry and Rumple head to NYC to find Nealfire
I think most of us agree that Neal got engaged to Tamara out of fear and thinking himself to be in love but really not. I could see him getting engaged quickly, ie: in between October and May. In my head he was already engaged when he got the postcard.