We have developed HyperCard software that runs on Macs for this purpose. We
bill once a month, but the invoices list the services and dates. Multiple
identical services on a given date (e.g. several sequencing samples) are
entered as a single line, indicating quantity. The software also produces
summaries that are useful for annual reports, etc. What I like about it is
that my assistants can simply click on buttons and fields to do billing and
extract summaries, and I can make the programming behind the buttons as
sophisticated as I want. It is easy to use and train, but an investment of
time to develop and program. I can give more details or provide a copy of
the hypercard stack if you are interested.
Vahe Bedian