Replacement for paypal in pro site

I dont want to use paypal on my site and have a alternate gateway.

the gateway link url is as follows
http://url.com/payment_amt=5&product_type=1

what i like to do is , when a user wants to upgrade from free to pro at that time depending upon the type of plan he has selected.

The url shown above should be auto updated with the values.

Can any one help ?