set form #action in hook_validate?

I'm finishing a module to capture new customer information for a client. The new cust will fill out the form which includes several taxonomy terms and either email the form to my client and send a check or pay via paypal. The email portion works fine which is handled by myform_submit. For the paypal, I've included paypal's hidden fields in the form and if I set the #action to the paypal url, clicking the pay now button immediately sends me to paypal. What I want is for the pay now to first run thru myform_validate so I can set the hidden fields using form_set_value. My client is selling monthly subcriptions of three types each at a different price and, based on the package selected on the form, I will set the correct price. I have tried to set the #action in myform_validate but it does not work.

If anyone can help me dynamically set the #action or has a suggestion as to how to approach this, I would greatly appreciate it.