The ACT! Schedule Activity entity should have the capacity to add a custom button to assist with any workflow process that naturally flows from scheduling or complating an activity. For instance often a business will either schedule a chargeable activity such as a professsional consulting activity or a even accept casual phone consultation, unscheduled. Logically the first is a scheduled Activity and the latter is possibly best treated as a History recording process.
There is no ready workflow process currently from that Activity or History leading to the recording of a sale. However the ACT! Opportunity entity does accept a list of items and associated pricing. If from either an ACT! Activity or a History, the Opportunity functionality could be accessed, that would provide a rudimentary but useful process of recording a sale which could be progressed to an invoice through an accounting link product, either now or later.