plugin extension travel agency

[estevan83]

hi guys ,
i need to customize virtuemart before add to cart an item. (travel agency)
Now I've modified the component but I would like implement an external plugin that do this work.
In this plugin I need to add some informations like travel date and the number of passengers and after them submition to an external web service, I receive the availability of travel and the price of it.
Only if this travel is available I can allow  the user to add to cart this travel.
Remeber I need also to modify the price that comes from external web service istead of default product price
How and which event I need call to intercept handler?
I need to create another event?

Thank in advance estevan83