What do you charge for add-on services, like watering a lot of plants, collecting mail, rolling garbage bins out, etc.?

I don't charge extra for doing the extras like you listed, when I am on a House stay. But that does seem odd they would ask you for a drop in visit.
I personally would probably still not charge them for the extra stuff but I also would make sure I was only staying the 30 minutes there. When the clients ask for extra, I would mention that you don't mind (example "watering") but that really takes away your time with their pet. Letting the owner know that you are only there for 30 minutes.
That might start detouring them away and make them realize you are really their for their pet.

( is it just me, or do you find it kind of rude they would ask you in the first place? I totally get it when they are away and you are staying in their home. Totally understandable, but when you do a drop in visit.... That normally means they are home at night (or someone is home to take care of their pets at night) so why would they ask you to take out their trash? (or what ever it is they ask un-pet related.) to me it kind of says I hate doing this chore so I will get someone else to do it but your not a personal assistant, your their dog sitter. ) and some may say, " it only takes 1 extra minute to take out the trash. But at the same time, if it does only take "1 extra minute" then why can they not do it themselves? I don't know, that would rub me the wrong way. ugh...

I totally agree with Erica about performing those little chores if you are house sitting. There's no real reason to do them for a drop-in for exactly the reasons Erica mentioned. Totally logically. If they want you to do it, then be sure they understand your first priority is the dog and you will only stay for 30 minutes. If they'd like longer, then create a rate for the additional stay time and have them book it that way.

I do understand their need for watering because of your heat wave though. However, they could get up early and do it before going to work or see if a neighbor's kid wants to earn some money.

Add: It is really insulting to think that anybody is going to do all those chores, plus tend to a dog, for $15.

I'm also asking if you ARE willing to do the extra chores for a fee, how and where to do add on the extra charges, and how much of that does Rover take?

I only do drop in visits, so I definitely get the mail and roll in the trash cans when the family is out of town. I will roll the trashcan up the driveways if I am just dropping in for a walk (when the family is not out of town) I do not get their mail. I've gotten a lot of tips by doing this without being asked. I actually have a rover customer who has me feed their outside cats, so that takes a total of 5 minutes. So the rest of the time, 25 minutes, I also water their plants, get their mail and clean their koi pond filters.. all within the 30 minutes though.