StefLea

I haven’t seen anyone post that, though, when it does come, it’s pretty short notice (a week or less). 

Got my letter in the mail yesterday.  Yay! 
Now for bio letter..... 
Has anyone ran into the problem of not receiving the letter and missed appointment? I'm hoping that this letter won't be an issue like the NOA. 

Finally I received the original NOA1 letter (with 18 month extension) today!!!  
 

Update : 
Did not receive Noa so I sent e-inquiry Feb 13th
Got email Mar 1 stating:
   'researched the status of your case. Our records indicate that on January 14, 2019, we sent you a receipt notice to the address on file at the time. The receipt notice has not been returned to us by  the post office as undelivered.  As a courtesy, we  will mail a duplicate receipt notice. You should receive the notice within 30 days.'
 
Really hope it's the official NOA... So frustrated with all this extra delay. 

No.
You do not need a stamp at all - there is no legal requirement to do so.
However should you want to - and it is entirely sensible to do so, for a multitude of reasons - you can get it before expiry, or after expiry.
Be aware that certain staff may get the hump if you ask for one more than say, 30 days or so before expiry, unless you have a pressing reason to do so (such as travel).

From browsing other filing months I know that the NOA letters are delayed. I might not get it before my green card expires. Do I need to get an infopass appointment before the card expires? Or can I wait till when I actually need to travel(July/August)?
 
Quick rundown on my filing so far. 
Jan 7 - mailed to Texas lockbox
Jan 14 - e-notification receipt SRC
Jan 15 - check cashed
 
GC expires Mar 9