What about just offering them straight hourly for the first month or two - this will give them a taste of how many hours a month they need AND your fabulous services
I've only ever invoiced on an hourly basis, have never offered retainer - but there are a few services we offer that are pitched to the client as a 'package' - in a case where the client wants to run with the service but can't budget for the full hours per week we suggest devoting to it we will always make an exception on the number of hours but never the hourly rate.