1. A method for paying with loyalty currency during online payment at any merchant website by a user using a payment assistant plug-in residing on a browser, the payment assistant plug-in being independent of any merchant website, the loyalty currency being provided by one or more program sponsors, the method comprising: automatically presenting loyalty currency balance information for the user corresponding to the one or more program sponsors, when the user visits one or more merchant websites; converting, using a computing device, total cost of items to be purchased into one or more loyalty currency units based on a set of one or more conversion rules stored in a conversion table; receiving a confirmation from the user to use the converted loyalty currency units for a purchase transaction at a merchant website; filling payment methods details using a payment method in a payment form based on the received confirmation; presenting a confirmation message to the user that a purchase transaction has been completed by paying with loyalty currency at the merchant website, irrespective of whether the merchant website supports paying with loyalty currency; and reporting purchase transaction information to at least one of the one or more program sponsors after completing the purchase transaction, wherein the method is implemented on a computing device.