Is there any way to select a product based on a form value? Is there any way to add a description to a product?

Unfortunately, the products in the payment integration cannot be tied on the conditional logic. Unless you'll use the quantity field in your products as the "IF" statement rule. 

If you have a complex pricing, you can recreate the products using plain form fields, assign values on them and pass the final price in your payment integration as a custom amount. 

Here are the guides that might help:

1. To assign calculation values on drop downs, radio button and check boxes, you can follow this guide: http://www.jotform.com/help/301-How-to-Assign-Calculation-Value 

2. To perform form calculations, you can use the form calculation widget as mentioned in this guide: http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form. Or use conditional logic.

3. Now if you have a calculated total price, you can pass it to your payment integration by following this guide: http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field 

If you need help on setting this up, please let us know.