I have decades of experience photographing everything from flat art to photos to book pages and covers to stamps and coins. The following assumes you want pro-grade images.

Here are the basics:

Keep your camera parallel to the envelopes (art, photos, film, whatever). Use a tripod or copy stand. Use the self-timer for two seconds to avoid vibrations (a real problem in macro work!).

Use two identical lights, one on each side of the art, equidistant, at a 45° angle. BE SURE they have a CRI of 95 or higher if you want the best color. I use a couple of Viltrox L-116t video LED panels (around $100 for the pair), but a pair of clamp lamps and 5500K LED bulbs from Home Depot will work okay in a pinch (I have used them). If your lights are adjustable like the Viltrox, choose maximum output and around 4400K. (That uses both the yellow and blue LEDs at close to full brightness.)

METER your lights all across the surface of your copy area. Get the exposure even to 1/6 stop if possible. You can use a hand-held meter if you have one, OR, photograph a sheet of white paper as large as the largest object you will photograph, and adjust the lights until the image looks perfectly even.

Use a short macro lens if you intend to do lots of this. They are FLAT FIELD, for extremely low distortion, and made to copy flat objects. On DX Nikons, that's a 40mm Micro Nikkor.

Alternatively, test the glass you have, and use the lens that gives you a combination of closest focus, least distortion, and sharpest image.

You need little to no depth of field, so stop down between two and three stops from wide open. That is most often the sharpest aperture on most lenses.

Set the lowest NORMAL ISO possible (no "extended range" ISO).

Compose your image. Focus. Then replace it with a photographic gray card (see below).

Adjust the shutter in FULL manual mode to get a normal exposure of a photographic gray card* (Delta-1 8x10 cards are around ten bucks at B&H or Adorama).

* The exposure is normal when you fill the frame or metering area with the gray card and make a test exposure that centers the spike of gray.

Use the gray card to perform a CUSTOM, or MANUAL, or PRESET white balance. Then check the exposure to be sure you have a narrow spike in the center of the histogram.

Do not change the exposure unless you raise or lower the camera relative to the art.

Copy it! Remove the gray card, put the art back, and fire away.

For quick-and-dirty work, JPEGs based on a custom white balance and manual gray card-metered exposure should be fine. If you want post-processing control, save raw files and post-process them.

Documents, maps etc are one thing, just the addresses on the envelopes is a much easier thing.

The addresses can be done with any lens that will focus close enough to get the address large enough and clear enough to read. Though a macro/micro lens would be best.

For documents, maps, old photo prints you need a flat field lens to avoid distortion - macro/micro lens are flat field. (the ones that go to 1:1, not the close up lenses the OEMs often label "macro")

I have known some people who do document photography to use an old enlarger lens modified to mount on their camera and then they mount the whole thing on an old enlarger frame and often use a laptop to tether the camera so they can see what they are doing in large detail. They do make special gear for photographing documents, photo prints and maps that are too large and/or fragile to run through a flatbed scanner*.

I took over the Senior Center Photo Club from a lady who got too busy with volunteer work at the city library - she is digitalizing all their old newspapers, magazines etc. She uses their old microfilm rig, macro lenses on cameras, scanners etc. and often for the old faded and yellowed paper has to run them through photo editing software to make them readable.

*for something like envelopes just using a flatbed scanner and then doing a quick crop and tweak in post processing software will do it.