Happy Birthday TheStath!
Yes it certainly does stink when our friends let us down and forget important things like our birthdays.
You mentioned that this person lives near a shop that sells cards and that the post office is a short walk and cards are not that expensive, all normal thoughts. This is where our relationships with alcoholics are different because their thinking is not normal and usually not very healthy. And for us to have any kind of normal expectations from them is not realistic.
In active addiction there is no room for anyone else except the addict, it’s all about them, their needs and wants will always come first no matter what. It’s hard not to take it personally but this person is not drinking at you or because of you or to make you feel sad and down. They are drinking because they are an alcoholic.