Why wait for a warning SMS?? Top up a small amount x 12 to push the validity date to a year away. Boontherm machines top up for 10 + 2 baht. So the whole exercise will cost no more than 24 baht + 120 baht of added credit.
No need to find the coins. You can use notes. Tap your number in to the machine, choose a 10 baht credit and then put in 100 baht when asked for 12 baht. Once completed, don’t expect change, just repeat the exercise. This time it won’t ask for the 12 baht, it will deduct from the 88 baht stored in the machine against your number. Keep repeating the exercise until you have done it twelve times. Should take just a few minutes at the machine. Orange boxes outside 7/11 and Family Mart.