How do you charge for drop-in requests with walks?

I can generally easily fit in a 10 to 15 minute walk in the allotted 30 minutes for a drop-in. I would push to schedule the visit you have described as a drop-in, since it includes feeding/watering/mail, but as long as both you and the client are on the same page regarding what is expected of you, it would be okay if they requested the visit as a walk (assuming your rates are the same for both services.)

If the client is going away, are you doing multiple visits per day? If so, have one be a traditional walk, and the other visit(s) be traditional drop in visits. Or you could have a short walk/drop in combo each visit.

I do only drop ins and walking. I charge the same price, as I've found it's too confusing for clients if I charge a different price for each. So, I tell them on initial contact they're essentially paying for the time with their pet. Walks and drop ins are both scheduled for 30 minute sessions. Whatever they want me to do want me to do in that 30 minutes is fine. I like how the new report card app works, because you can record a walk as an "event" that occurs during a drop in visit. All with the 30 minute window. So if you're also feeding, watering plants, etc. all those activities can be done in the same 30 minute window.

You can tweak your "per visit" rate before booking the stay. Raise the daily rate to what you would like to charge them for including walks, and then book.