I would expose a text input and a button to the user. They would fill in the value they want to pay on the subscription, then when they hit the button your script would fill in the appropriate hidden field(s) on the PayPal form. In this case you might change the "a3" field's value to match what the user had typed. Then your script could submit the form, which posts the updated amount over to PayPal.
Once you have that down, just expose a few more fields for the user's info (name, etc.) and do the same thing on the hidden fields before submitting the form.