As we cannot nest forms, in your paypal code, you need to remove the opening and closing form tags.
Then change the button so that it modifies the form post url and then submits the form instead of just submitting normally. In your button, create an onClick property like this
replace ...... by whatever was in the original form tag for the url (I do not rememeber the specifics.
Note: This is what I remember from many years ago, not sure if paypal forms are still like that, if not, give us a code sample please.