Cash Tips

When a customer tries to hand me a cash tip, i say to the customer what i always say "oh no you are good the tip in the app is enough". They always give me the tip... until tonight. The customer tonight offered me a cash tip and when i told him he was good cause he tipped in the app, he was like "you sure man?" I said yes you good but i appreciate it (hoping he insists). Anyway tonight it backfired . What made it worse was when i got back in my car drove up 2 blocks to look at the order. That's when i realized he didn't tip in the app.