You want to make this as easy as possible for anyone writing a letter for you. Several options:
1. best option: many schools have a letter service. Your professors send their letter ONCE to the letter service, which then copies and forwards the letters to each school as necessary. Your premed advisor should be able to tell you if this service exists. It is absolutely worth paying for, even $4/letter. You know which letter went out, where it went, when, etc.
2. next best option: use Interfolio. Same as above, but a company rather than your school.
3. less desirable: you provide a stamped, addressed envelope to each professor. Problematic because you won't know when they send the letter, and they might forget. Also problematic because it is a LOT more work for them to send a letter each time you need to do another secondary. You ideally want a dept letterhead envelope, so you could provide a sticky address label and a stamp. All around pain in the ass.
4. least desirable: you provide the envelope, they seal it, you pick it up and mail it. Or, they provide letter sealed in envelope, you pick it up and mail it.
5. not an option: they give you the letter without an envelope (unless you have retained your right to see the letter).