Sooner you try the more likely you'll be able to.
Yes you will be able to pay for checking in one bag at the airport. Might be even more expensive than including it in the ticket obviously.
BUT ryan air allows really big hand luggage actually... You can't bring razors but you can bring liquid, they must be in a plastic bag and less than 1 litre togeher and 100 ml each (i think this is the restrictions.. says on their webpage), but you'll definitely be able to bring some face moisturiser, deo, make up etc..
If it's just cosmetics you are worrying about it will probably be cheaper to buy razors, deodrant and whatever else women need when you are there than to pay £24 for the privilidge.
Call them/go to the website. My flatmate did the same thing and they let her pay after saying she wasn't taking a case. I *wouldn't* leave it until you arrived at the airport - that could go horribly wrong.
You can usually bring luggage along when you check in and pay for it then, though it may be more expensive then than doing it beforehand.
Do easyjet charge for luggage too?
Go on the website and add it before you go.
I think it's more expensive than if you did it while buying the ticket, but cheaper and more convenient than adding it on at the airport.
I wouldn't phone them, their phone line is really expensive and from experiences my friends have had they don't actually answer the phone. Anyway you don't need to, you can either book a bag online (there will be an option somewhere for you to add a bag) but you really don't need to- as other people have said, just take travel sized stuff on in your liquids allowance. They don't actually check there is only 10 things in there as long as the bag will shut properly.
Yes you can take hair straighteners in your hand luggage, all electrical equipment such as phone chargers etc is fine. I do it all the time and I travel by plane at least once a month.