Both sites, Seamless and GrubHub (they're the same company) make it really easy to figure out the percentage based on the order total, and offer the choice to either tip in cash or credit. Therefore, according to Grubhub's website, if everything goes smoothly with your delivery and the weather is good, 20% is the standard cost of having the mealtime brought to your doorstep. Never tip less than five bucks, though. We can't explain it any better than Grubhub themselves, 20 percent is the standard if all went well.