Updating Recurring payment profile in rails

I have implemented Recurring paypal billing in my rails application. It is going on well. I can able to update amount and can cancel profile every thing.

Now my problem is, when a user upgraded his package, I would like to charge him the extra amount along with updating the actual amount for the new package. I have strucked with this. Can any one help me.

Thanks开发者_JAVA技巧 in advance, Vijju

"Updating the Billing Amount

For profiles created using Express Checkout, you can increase the recurring payment total amount by 20% maximum in a fixed 180-day interval after profile creation. The 20% maximum is based on the total amount of the profile at the beginning of the 180-day interval, including any shipping or tax amount.

If, for example, you create a profile on March 10 with a total amount of $100, during the 180-day interval from March 10 to September 6, you can increase the amount to a maximum of $120 (120% of $100).

Suppose that during the first 180-day interval, you increased the payment amount to $110. During the next 180-day interval (starting September 7), you can only increase the amount of the payment to a maximum of $132 (120% of $110)."

https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_ECRecurringPayments