Pre-book a half-day photo session at your location/venue/premises. This charge can form the deposit for the complete session or could even be the total price. This would depend on the nature, format and location of the commission or the extent and nature of additional services. Arrangements for the photo session can then be made via email or by phone, or preferably via the contact form system..
If this payment does not cover the full price of the commission it would act as a deposit and would be deducted from the final invoice.