Lubridate summarise on a nonfixed “monthly” basis

Question

I have the following data:

Customer <- c(1,2,3,4,5,1,2,3,4,5,1,2,3,4,5,1,2,3,4,5,1,2,3,4,5,1,2,3,4,5,1,2,3,4,5)
Invoice <- c(100:134)
Invoice.Date <-