Not sure why they wouldn't do it, maybe they where just helping you out. Your first covid extension starts from the day you apply, it's not added to the end of your current stay like other extension. It wouldn't make sense to apply for it early.
Subsequent covid extensions are added to your current stay, so you can apply for those 30-45 days before your stamp expires depending on the office.
Assuming you're entering on a 30 day visa exempt entry, you can apply anytime, the 30 day extension will be added to the end of your current stay. Cost 1900 baht, you need copies of your passport, entry stamp, tm6 and tm30.