I've created a few forms on this page http://maximusmartialarts.com/register/
On the beginner tab, I have three subscription fields at the bottom all the same price of $99. the difference between each one is the length of subscription (3, 6, or 12 months) When I select an option, It simply goes to the confirmation, but doesn't send the visitor to paypal.
If the amounts are different, then it seems to work.
You can download a copy of the form here...