The above link can be used when you wish to send an email to a patron to have them pay for the order online.
Parameters:
if you add the order number using order=xxx then the specific order will be automatically added to the cart. It means you can put a link in an email merge to pay for a specific order if you want.
This will land the patron on the Order Balance web page (after they log in) so that they can select the order to pay online.