Method_Jennifer:I am wondering how long it would take to do the following customization, and how difficult would it be.
As support staff we would not be the best folks to quote timing on a customization job like this - best bet for a quote and timing on your job would be to speak with a consultant on staff here.
Everything you posted is possible -
Method_Jennifer: The client would like to search their database for their customers who have purchased an initial service
Provided the information on their initial service is stored somewhere in Method then this would be a simple retrieve value from table action to see if anything exists. If it does then you know they got their initial service and you can trigger another action which will create an activity to contact the customer for a second service.
Method_Jennifer: They would like to see those who have not purchased in a while
This would be a retrieve value from table plus the date difference action to figure out how long from today their last purchase was. The invoice search would be another action which is looking for invoice line items to see what was purchased.
Method_Jennifer: For example, they would like to search all clients who have bought the Initial Service for $99 but haven't signed up for any other services.
Combining the actions I posted along with a custom screen what you're asking on behalf of a client is definitely possible and is something they should consider talking to a consultant on staff here to iron out all the details.
~C