Can't really help you but I assume this is a personal opinion. Some folks wouldn't tip at all and others would treat like at a restaurant or for a taxi.
I would lean on the low side as I consider this part of the service.
I ordered from www.gardengrocer.com. I saw several referrals for them on the DIS. There is another one, also mentioned, but I can't remember the name, or where I saw the thread. I'm sure someone can help.
Cheaper than the DVC order form, and more to choose from. You just shop my category, fill out your info, billing, and your resort and date and time you want it. You do have to order early enough that they are not already filled up for that date, however.