redirect to paypal from action not view

I am developing an e-commerce website and I want to use paypal as a payment method.

I have look at Pro PayPal E-Commerce book at it was really good.I am using asp.net mvc

If I put my fo开发者_StackOverflow中文版rm in the view ,every thing work fine:

but I want to do some validation before sending the data to paypal ,so i like to go to one action first do some validation and then redirect to paypal so the customer can buy the product. I am really confused should I use Httpost or just a redirect

Thanks Elham