PayPal Integration with PHP site for one product

We have existing PHP site with a minimum work on PHP and mostly static using HTML. We would like a new checkout page for a site where customers will fill in a small form which will dynamically calculate cost depending upon form information. The payment is then processed through paypal for credit card payment. Once payment is successful we need to call an API to create user account from same details originally entered by client.

(API code and cost calculation formula will be provided)

So In summery we require form filled in and cost calculated on our site, then payment processed on paypal, once payment is successful user to be created using API and all details saved in a database.

there is only one product but with different options hence calculation is required for final amount.