I have only 2200 points in my United Airlines Mileage Plus account and they were going to expire in a month. The only reason I was aware of it was I got the magazine subscription with points in the mail. I didn't want to waste my points and I also don't need any magazines. Turns out I can quickly extend my points for another 18 months by use the Mileage Plus X app on my phone!
Basically you buy these gift card through the app to use them. I was going to buy an Amazon gift card for $20 but turns out for Amazon has only predefined amount starting at $25. When I log in to my account, I see exactly the point balance and the expiration. After I bought the amazon gift card, I added it to my amazon account right away and the amount deposited there. I forgot to check my mileage plus expiration until a couple days later but voila my points expiration has been extended for another 18 months.