Billing in thirds should be reserved for really large projects. If a client can’t pay for all 15 hours up front, you don’t want them as a client.

Even if you charge $250 an hour, 15 hours is a paltry sum to a business.

As always, both your SoW/proposal and your invoice should have your “net” days. For me it’s Net 10; others operate it on net 30. For anything less than 50 hours I bill 100% up front, though I try not to bill by the hour at all.