This one's unusual but not unheard of - you can buy specialist DVDs of women sitting on and squashing big balloons or blowing them up until they burst.
You need to take all these sorts of enquiries with a pinch of salt - if they make an actual concrete booking then all well and good, but a lot of the time these emailers are just fantasists who get off on describing their various kinks to you; I get similar ones including some who have even gone on to make appointments but they have always cancelled. I know it's tempting to take every single email seriously as a potential booking and you should certainly reply and follow up, but if you can't get anything approaching a firm arrangement after this then I wouldn't waste any more of time on it. Don't forget, they know you're new!
I got a very polite call once from a gentleman who wanted me to sit on him and force feed him worms and snails and a mouse (I hung up when he asked about live goldfish). That'll teach me to advertise in the Daily Sport lol.