Hi,
There is a drop down menu and two radio buttons in my form.
The drop down menu is to select the product and radio buttons are to select whether the payment method is moneyorder or paypal.

So, when user selects product A from the drop down and paypal as the payment method as radio button then the thankyou page should show the buy now button for product A from the simple commerce module from EE. This can be done with IF Else statements easily. But I am unaware on how to notify the thank you page that what things are selected in the form.

Any help will be highly appreciated.

Thanks in advance

Perhaps this will help:

http://www.dynamicsitesolutions.com/javascript/hiding-and-unhiding-table-rows/