Not entirely sure what ketchup or envelopes you’re getting that have wheat, but Heinz is free from gluten. The vinegar they use is corn-based as well if that sketches you out. I get Sir Kensington because it’s the bee’s knees and tastes better.
In the US at least, stamp & letter glue is corn starch based as well. No gluten. Source
The biggest surprises for me were soy sauce (which you can replace with tamari) and the sheer number of things that contain barley malt. Not all malt is from barley, but most of it is. That, and anything that uses shared equipment usually makes me itchy.