You should have been well trained to check ID's.
You broke the law, pay the fine.
Employer may want to fire you for the added cost of having you as an employee.
If this looks likely, you may want to consider offering to have $50 per week deducted from your paycheck to cover the fine that you cost employer.
If employer did not stress importance of legal age requirements on products, then suggest that this is good time for extra training to all employees.
I used to get threats or bribe offers all the time while enforcing sales laws. You have to be prepared with good come backs. If told, I left house w/o ID, tell them you'll be glad to hold product until they return & remember to always bring an ID when you want to make this purchase. (Be friendly) Let them know that you have to up hold law. Or you can offer to call P.D. & wait for them to come with advice. (Most won't wait b/c they know what answer you'll be given.) I'd be offered "a little extra money if I can do this for them." I'd say Ok, "That will be $X for item, $300 for my fine, $3000 for my employers fine, $6000 to cover my unemployment after I get fired so that I can find a new job. That comes to $9,310." They get indignant of course, but I tell them it's a fair price for asking me to break the law. They'll say, "No one is around to see." Tell them you are & I have no resources for your background check to make sure that you aren't setting me up. It's just like me asking to live in your house for the week. You don't know me & how many of your possessions I will take when I leave. You are wanted me to trust a stranger over the law I KNOW will prosecute me. Your asking me to put my job on the line for your cigarettes or beer. I don't know you that well. Who are you do ask me to lose my job for $10 bucks?"
No matter what they say, keep referring back to your key points & make them answer your questions. If they get really frustrated, refer them to a shop down the road who may be able to meet their needs.