All self checkout registers have someone standing near the self checkouts monitoring the self checkouts so you can purchase alcohol through the self checkouts.
When you scan the alcohol at the self checkout the register will alert the person monitoring the self checkouts and then they will come over and enter a password and check your ID to see if you're of age to buy the alcohol.
If you're of age to buy the alcohol then the person will enter the password and confirm that you're of age to buy the alcohol and the alcohol sale will go through.
I've bought alcohol through self checkout register and it's never been a problem.