Inbound (arrival day):
At your home airport:
If you use curbside check-in, a tip to the skycap is customary when the skycap hands your boarding passes and baggage claim receipts to you.
If you check in with an airline counter agent, you do not tip -- even though the airline counter agent performs the same functions as the curbside skycap.
In either case, be sure to attach your yellow Disney's Magical Express (DME) baggage routing tags to any bags that you check.
At WDW:
If a DME driver has stored your carry-on bags in the luggage hold of your DME motorcoach, a tip to the driver is appropriate when he or she returns your bags to you upon arrival at your WDW resort hotel.
If your carry-ons are just personal items that you can keep with you on the motorcoach, you should not feel an obligation to tip. If the driver is exceptionally pleasant or provides personal service, you might choose to tip anyway.
You do not need to tip when bell services brings your bags to your room. Disney takes care or the compensation for the bell services CM when DME is used as designed. Chances are you won't be in your room when the bags are delivered.
Outbound (departure day):
At WDW:
If you use Resort Airline Check-in (RAC) to check your bags and obtain your boarding passes, a tip is customary. Think of RAC as curbside check-in, except that the curb is 20 miles from the airport.
If a DME driver has stored any of your bags in the luggage hold of your DME motorcoach, a tip to the driver is appropriate when he or she returns your bags to you at the airport.
In addition, if you use the services of porters at your home airport, porters at Orlando Airport, bell services CMs at WDW to assist you (beyond the basic inbound delivery of your yellow-tagged bags), the bell desk to hold your bags until your room is ready, the bell desk to hold your bags until your outbound DME motorcoach pick-up time, or other services that require additional handling of your bags, tips are usually expected.
Using DME transportation and baggage transfers, and handling your own bags at other times, you can minimize how much you need to
budget for tips.
As far as the amount is concerned, start with the "$1-$3 per bag" rule of thumb. Then consider the friendliness and helpfulness of person helping you. If I use a skycap service, I typically tip a $5 bill regardless of whether I check one, two, or three bags. The amount of work is essentially the same for the skycap. I will go higher for exceptional device. I would consider going lower if the service is marginal, but I usually find skycaps to be pleasant (after all, they rely on tips).
There is no need for envelopes.