Have already implemented a solution in vb.net and javascript (html page) to make payment on alertpay from existing website
The website was referral website so when someone joins in, he has a pay button which shows a screen :
Please wait for AlertPay login page
which is followed by the alertpay website login pay with the amount to be paid and to whom to pay
When he makes the payment, it says payment done and he is redirected to the original website with a return value paid or not
So everything is done already
just need to set it up with your existing pages
Thanks to confirm