Having the Access Token, you can access all protected Endpoints, if the user has an access to them. Note that Bills Payments don't use the iHela default authentication describded here before.
iHela uses bills payments system to also performs all two steps merchant payment logics. It assumes that the application has to create a Bill via the iHela API and allow the user to pay it. iHela links to a bill the redirect_uri parameter to allow redirection any time the user decides to pay the bill.