£300 is a hell of a lot to pay for sex, no matter how good it is. For that you should be very sure it is going to be an amazing experience. By this I mean you are already in a raunchy session with her, she is doing things like kissing you, sucking you off and you can see she is in to it already. The fee is to upgrade to FS, or extend time, or both, and you are fairly certain the session will continue in same vein. Even better is you have had a shorter session with her previously and were very happy with the service. Or you have read multiple positive reviews on UKP from members you trust.