my sister just asked me this last night. she had thai delivered, cost about $22 and she also tipped $3. (it was a delivery service that came to the door, not someone from the actual place, which is a few blocks away)
I always get flustered for some reason when the delivery person comes, especially if i didn't know the total beforehand. Figuring out the change and the tip on the spot. I usually do the calling and someone else in the house does the door.