Not a low fare guarantee, but I am waiting on a refund of a partially cancelled ticket. I did it last Tuesday (over the phone as it could not be done online) and it still hasn't shown up in my wallet. In the past, low fare credits and refunds have always posted pretty much instantly but I did a search here and it seems it is not unheard of for it to take a week or so for the funds to hit my wallet. I figured I'd call if the credit wasn't there by this Tuesday.