Could someone please help with suggesting a plugin ... done looking : / - (a form+cart) bundle !

I have been searching here and there, came across many form plugins and store payment plugins, but Im looking for a system that would let me do both things at once: a form+shopping cart plugin.

Im looking to build an order system similar to this brilliant system on their order page(without the spaces):

www dot writemyessay dot com/custom-writing/

It is a form that lets you specify how many pages of writing you need and then based on the due date, uses a rate to calculate a total and lets you check out immediately...

Im hanging my - cannot find something like this .... any help please??