What happens if I apply for a Thai visa extension late and pay the fine?

Go on time for your extension or you could be denied.

The 18th is a Sunday, so you'll probably need to go on the 16th to get your extension.

See what date they give you on the extension, then decide if you want to overstay and pay the fine or do it legally and get another extension or do a border bounce.

Whether or not it will affect your holiday in December is hard to stay. They might be especially picky in terms of onward ticket, having 20k cash available, etc. if they see you've overstayed in the past. Might be safest just to pay your money for the bounce/ second extension and save the stress in the future.

If you go late, you could get deported. You'll be in the country illegally. Unlikely but a possibility.

Marco is wright !

No, you can get a 30-day extension (1,900 baht) AND a further 7-day extension (1,900 baht again) at Immigration

Extension is from the date your visa expires so, no. You could do a border run part way through your extension to ensure you will have enough time left and you are staying in the Kingdom legally.