>I've been floundering around with billing for a while now, and made a few posts. I think, though, that I was to general in what I was asking. After giving it some serious thought, I think I've come upon what is the major problem I'm having. How do you deal with past balances?
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>Without past balances, I can make the invoices for any time period and figure what was owed at the end of that time just by adding up the trip prices. But the client balance file is always changing. If I want to bill up to July 31, but don't actually run the bills until August 5, any trips or payments entered 8-1 to 8-5 affect the balance.
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>I pseudo-solved that by saving the balance file July 31. But what if you save it then, enter trips up to the 5th, and then discover you missed a trip from July 28th? It's too late to resave the balances.
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>I need a way to make the billing more dynamic, to allow editing of invoices, adding more trips when necessary, changing the prices of trips.
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>Somebody out there must have had to deal with past balances. Help me, please?

I never work with either saved files (too often you find you missed entering some data, and it´s a bother including it in the saved files) or accumulated totals (if a record with a previous date is entered, the totals are wrong).

I consistently use SQL SELECT sentences to calculate totals up to any date; since you don´t do it often, speed doesn´t matter much, and, much better, it can´t be wrong.

There are techniques that allow both for SQL processing and precalculated totals, but they are more involved and, frankly, not needed unless you have serious performance requirements.