What would you tip?

Okay... I'm ordering a pizza online for it to be delivered. I'm in a small town so it will only take them about 6 minutes to drive to my house. I usually do pick up but my husband is at work and I had a hard day at work so I'm rewarding myself with pizza. It's only one pizza but the total is $18.07 (including tax). We usually tip 15-20% when we go out. How much would you tip? There is already a delivery fee of $3.50. I'm not sure if that is a tip for the driver or not but would $3 be okay? That would be about 17%. I feel like that's low but it's more than 15%, they're very close by, and it's one pizza. I'm just so unsure! Any advice?

Update: I had no idea the delivery fee didn't go to the driver!! What the heck?! Why is there a delivery fee when there's no carry out fee and they're not using it to pay the driver?! That is such bullcrap. I mean, I was already going to tip anyways but gosh that seems like a crazy thing. Thanks for the advice! I'm going to do $4. I feel a lot better as I kept feeling like I should be doing $8 or something but that seemed high percentage wise. My husband is a bartender so I fully understand the tip is basically what they get paid, he only makes $2.50 an hour without tips