Integrate Custom Form into Checkout Process

[things1did]

I am trying to integrate a custom form into my checkout process, but do not know where to start.

My business is a personal shopper, and my site does not sell individual products.  Instead, I charge per item delivered.  For example, if a person receives 6 items from me, then their bill would be $6 + the cost of the goods purchased.  The cost of the goods purchased is not known when they place the order on the website.

The flow of the checkout process is defined below....is this possible in virtuemart (the list below does not include every little step, such as entering a delivery address, etc.)?

1.) Fill out a custom form with detailed description of each item
2.) Click a check-out button on the form
3.) Based on the number of items on the form, a virtual product is selected from virtuemart [4 items, 12 items, 3 items, etc.]
4.) The person then checks out as normal, but their credit card is only pre-authorized at this time.
      a.) Their check-out total would read "$6 (6 items) + the price of goods"
      b.) Discounts / coupons would be applied to the virtual product, as necessary
5.) I fill the order and then charge their credit card for the full price of goods + the $6

Is this possible within Virtuemart?  If so, how do I do it?  I cannot find any type of embedded form option, although I was led to believe it is available.\\

Thank you for your help!
- things1did