Starting early with record keeping can make life easier. In addition to tracking when companies post shops, also Who shops whom, pay method/cycles, and contact info.
Regarding being contacted by schedulers for shops, that can happen when assignments need re-shopped, following a rejection or flake. They will also contact shoppers to do a shop with a tight time frame, usually toward the last third of the month. I like to think these circumstances are when a shopper's reputation comes into play.