Can you say a little bit more about this?  It seems (from googling a little bit) like if you hit SaferPay's integration url with the right parameters, you get a form that does the payment.  I.e. you can literally type in to your browser something like
http://integration.saferpay.com/integrate?AMOUNT=0.23&CURRENCY=USD&....&SUCCESSLINK=...
and saferpay would render the correct form for customers to use.
If that's really the case, I don't think you need the `requests.get`, the redirect or this `process_purchase_callback` at all.  You can just move most of this code into `render_purchase_form_html` and have that function return an HTML form with action=the integration point and method=GET, and that should work.