you cant really do conditional summary the wa you have it
if datediff('m',startdate,caceldate) in 3 to 7...
you can instead apply the condition to the formula and sum that formula
if datediff('m',{servicessetup.startdate},{{servicessetup.caceldate}) in 3 to 7 then (({InvoiceLines.Quantity} * {InvoiceLines.UnitPrice}) *(.75)) else 0
now sum this at the client footer