My part time job as a retiree is lunch time pizza delivery. Half of my customers don't tip at all. I keep a log, and the average last year was 5% on tips. The night guys doing residential do much better. A couple of my regular large corporate customers have orders from $100 - $300 at a time. They have accounts so just sign for it. Zero tips ever. I still try to maintain a professional attitude, and provide good service even though I know I'll get stiffed.
Explain to the corporate guys that the tip is not included or make sure the receipt has a place to add a tip that is prominently noted. Or add 15% automatically on large and or corporate orders.
I do numerous large orders for my sales job. Many delivery guys make it impossible to tip by not accommodating a charged tip, not having enough change, or leaving it with the front desk.